Find files larger than 15MB:

find -type f -size +15M

Find files less than 12KB:

find -type f -size -12k

Find files exactly of 12KB size:

find -type f -size 12k

Or

find -type f -size 12288c

Or

find -type f -size 24b

Or

find -type f -size 24

General format:

find [options] -size n[cwbkMG]

Find files of n-block size, where +n means more than n-block, -n means less than n-block and n (without any sign) means exactly n-block

Block size:

1. c: bytes
2. w: 2 bytes
3. b: 512 bytes (default)
4. k: 1 KB