I was pretty certain that the answer would be "decimal", but I nevertheless went back to check out the BusyBox source code for the 'ls' applet (file: 'coreutils/ls.c', function: 'list_single()') where the following code snippet can be found:
...
if (all_fmt & (LIST_SIZE /*|LIST_DEV*/ )) {
if (S_ISBLK(dn->dstat.st_mode) || S_ISCHR(dn->dstat.st_mode)) {
column += printf("%4u, %3u ",
(int) major(dn->dstat.st_rdev),
(int) minor(dn->dstat.st_rdev));
} else {
...
So it is quite obvious from the use of the '%4u' and '%3u' that both values are formatted as unsigned (decimal) integers.