This is probably the info he was linking to in the discord(edit: there is a channel where people discuss portrait packs):
the slave tab portraits are 139x139
the slave tab full bodies are 370x608
the inventory panel's bodies are 376x430
combat portraits are 105x105, though enemies are 90x90(edited)
for sex interaction portraits are 35x35, and bodies are 600x600
thus the slave tab dimensions are basically the max you can use(unless you want something grand for the sex portrait), anything different is fine as it should keep aspect, though it will shrink/expand to fit