デフォルトは2つ分ですが、4つのほうが見やすいので。

nvimlspの設定

設定自体はnvim-lepconfigを使っています。
https://github.com/neovim/nvim-lspconfig/blob/master/doc/server_configurations.md#verible

space幅の設定

formatterをファイル管理することはできないみたいです。
language serverの説明が以下のリンクにあります。
--helpfullコマンドを使うことでいろいろな設定項目が見れるようです。
https://github.com/chipsalliance/verible/blob/master/verilog/tools/ls/README.md

その中で--indentation_spaceという項目があります。
これを設定すれば良いことがわかるため、lspの設定の中で以下のように設定すれば良いです。

                cmd = {
                    "verible-verilog-ls", "--indentation_spaces=4"
                }

補足

linterの設定

どうやら.rules.verible_lintをプロジェクトのルートディレクトリに置くことでできるみたいです。

--helpfulの結果

% ./verible-verilog-ls --helpfull
Verible Verilog Language Server built at v0.0-3617-g4f98e145
commandline: ./verible-verilog-ls --helpfull
verible-verilog-ls: ./verible-verilog-ls

  Flags from common/formatting/basic_format_style_init.cc:
    --column_limit (Target line length limit to stay under when formatting.);
      default: 100;
    --indentation_spaces (Each indentation level adds this many spaces.);
      default: 2;
    --line_break_penalty (Penalty added to solution for each introduced line
      break.); default: 2;
    --over_column_limit_penalty (For penalty minimization, this represents the
      baseline penalty value of exceeding the column limit. Additional penalty
      of 1 is incurred for each character over this limit); default: 100;
    --wrap_spaces (Each wrap level adds this many spaces. This applies when the
      first element after an open-group section is wrapped. Otherwise, the
      indentation level is set to the column position of the open-group
      operator.); default: 4;


  Flags from external/com_google_absl/absl/flags/parse.cc:
    --flagfile (comma-separated list of files to load flags from); default: ;
    --fromenv (comma-separated list of flags to set from the environment [use
      'export FLAGS_flag1=value']); default: ;
    --tryfromenv (comma-separated list of flags to try to set from the
      environment if present); default: ;
    --undefok (comma-separated list of flag names that it is okay to specify on
      the command line even if the program does not define a flag with that
      name); default: ;


  Flags from verilog/analysis/verilog_linter.cc:
    --rules (Comma-separated of lint rules to enable. No prefix or a '+' prefix
      enables it, '-' disable it. Configuration values for each rules placed
      after '=' character.); default: ;
    --rules_config (Path to lint rules configuration file. Disables
      --rule_config_search if set.); default: "";
    --rules_config_search (Look for lint rules configuration file
      '.rules.verible_lint' searching upward from the location of each analyzed
      file.); default: false;
    --ruleset ([default|all|none], the base set of rules used by linter);
      default: default;
    --waiver_files (Path to waiver config files (comma-separated). Please refer
      to the README file for information about its format.); default: "";


  Flags from verilog/formatting/format_style_init.cc:
    --assignment_statement_alignment (Format various assignments:
      {align,flush-left,preserve,infer}); default: infer;
    --case_items_alignment (Format case items:
      {align,flush-left,preserve,infer}); default: infer;
    --class_member_variable_alignment (Format class member variables:
      {align,flush-left,preserve,infer}); default: infer;
    --compact_indexing_and_selections (Use compact binary expressions inside
      indexing / bit selection operators); default: true;
    --distribution_items_alignment (Aligh distribution items:
      {align,flush-left,preserve,infer}); default: infer;
    --enum_assignment_statement_alignment (Format assignments with enums:
      {align,flush-left,preserve,infer}); default: infer;
    --expand_coverpoints (If true, always expand coverpoints.); default: false;
    --formal_parameters_alignment (Format formal parameters:
      {align,flush-left,preserve,infer}); default: infer;
    --formal_parameters_indentation (Indent formal parameters: {indent,wrap});
      default: wrap;
    --module_net_variable_alignment (Format net/variable declarations:
      {align,flush-left,preserve,infer}); default: infer;
    --named_parameter_alignment (Format named actual parameters:
      {align,flush-left,preserve,infer}); default: infer;
    --named_parameter_indentation (Indent named parameter assignments:
      {indent,wrap}); default: wrap;
    --named_port_alignment (Format named port connections:
      {align,flush-left,preserve,infer}); default: infer;
    --named_port_indentation (Indent named port connections: {indent,wrap});
      default: wrap;
    --port_declarations_alignment (Format port declarations:
      {align,flush-left,preserve,infer}); default: infer;
    --port_declarations_indentation (Indent port declarations: {indent,wrap});
      default: wrap;
    --port_declarations_right_align_packed_dimensions (If true, packed
      dimensions in contexts with enabled alignment are aligned to the right.);
      default: false;
    --port_declarations_right_align_unpacked_dimensions (If true, unpacked
      dimensions in contexts with enabled alignment are aligned to the right.);
      default: false;
    --struct_union_members_alignment (Format struct/union members:
      {align,flush-left,preserve,infer}); default: infer;
    --try_wrap_long_lines (If true, let the formatter attempt to optimize line
      wrapping decisions where wrapping is needed, else leave them unformatted.
      This is a short-term measure to reduce risk-of-harm.); default: false;
    --wrap_end_else_clauses (Split end and else keywords into separate lines);
      default: false;


  Flags from verilog/parser/verilog_parser.cc:
    --verilog_trace_parser (Trace verilog parser); default: false;


  Flags from verilog/tools/ls/symbol-table-handler.cc:
    --file_list_path (Name of the file with Verible FileList for the project);
      default: "verible.filelist";

  Flags from verilog/tools/ls/verilog-language-server.cc:
    --variables_in_outline (Variables should be included into the symbol
      outline); default: true;

Try --helpfull to get a list of all flags or --help=substring shows help for
flags which include specified substring in either in the name, or description or
path.