find with size

+ - bigger than size

sudo find / -size +100k

- - less than size

sudo find / -size -5M

Invoke size multiple times to specify size interval.
To show files with size between 100K and 5M:

sudo find / -size +100k -size -5M

If you want to specify OR betweeon the size options use -o.
To show files with size less than 100K OR bigger than 5M:

sudo find / -size -100k -o -size +5M