10進数の文字数と、16進数の文字数の比較表です。
（末尾に 'L' や 'LL' が付く場合は考慮していません）
10^13 から16進数の方が短めになるようです。

10進数				16進数
0	1	<	3	0x0
9	1	<	3	0x9
10	2	<	3	0xa
15	2	<	3	0xf
16	2	<	4	0x10
99	2	<	4	0x63
100	3	<	4	0x64
255	3	<	4	0xff
256	3	<	5	0x100
999	3	<	5	0x3e7
1000	4	<	5	0x3e8
4095	4	<	5	0xfff
4096	4	<	6	0x1000
9999	4	<	6	0x270f
10000	5	<	6	0x2710
65535	5	<	6	0xffff
65536	5	<	7	0x10000
99999	5	<	7	0x1869f
100000	6	<	7	0x186a0
999999	6	<	7	0xf423f
1000000	7	=	7	0xf4240
1048575	7	=	7	0xfffff
1048576	7	<	8	0x100000
9999999	7	<	8	0x98967f
10000000	8	=	8	0x989680
16777215	8	=	8	0xffffff
16777216	8	<	9	0x1000000
99999999	8	<	9	0x5f5e0ff
100000000	9	=	9	0x5f5e100
268435455	9	=	9	0xfffffff
268435456	9	<	10	0x10000000
999999999	9	<	10	0x3b9ac9ff
1000000000	10	=	10	0x3b9aca00
4294967295	10	=	10	0xffffffff
4294967296	10	<	11	0x100000000
9999999999	10	<	11	0x2540be3ff
10000000000	11	=	11	0x2540be400
68719476735	11	=	11	0xfffffffff
68719476736	11	<	12	0x1000000000
99999999999	11	<	12	0x174876e7ff
100000000000	12	=	12	0x174876e800
999999999999	12	=	12	0xe8d4a50fff
1000000000000	13	>	12	0xe8d4a51000
1099511627775	13	>	12	0xffffffffff
1099511627776	13	=	13	0x10000000000
9999999999999	13	=	13	0x9184e729fff
10000000000000	14	>	13	0x9184e72a000
17592186044415	14	>	13	0xfffffffffff
17592186044416	14	=	14	0x100000000000
99999999999999	14	=	14	0x5af3107a3fff
100000000000000	15	>	14	0x5af3107a4000
281474976710655	15	>	14	0xffffffffffff
281474976710656	15	=	15	0x1000000000000
999999999999999	15	=	15	0x38d7ea4c67fff
1000000000000000	16	>	15	0x38d7ea4c68000
4503599627370495	16	>	15	0xfffffffffffff
4503599627370496	16	=	16	0x10000000000000
9999999999999999	16	=	16	0x2386f26fc0ffff
10000000000000000	17	>	16	0x2386f26fc10000
72057594037927935	17	>	16	0xffffffffffffff
72057594037927936	17	=	17	0x100000000000000
99999999999999999	17	=	17	0x16345785d89ffff
100000000000000000	18	>	17	0x16345785d8a0000
999999999999999999	18	>	17	0xde0b6b3a763ffff
1000000000000000000	19	>	17	0xde0b6b3a7640000
1152921504606846975	19	>	17	0xfffffffffffffff
1152921504606846976	19	>	18	0x1000000000000000
9999999999999999999	19	>	18	0x8ac7230489e7ffff
10000000000000000000	20	>	18	0x8ac7230489e80000
18446744073709551615	20	>	18	0xffffffffffffffff