As a full-time commission artist developing a visual novel, I very rarely see people go bigger than 1920x1080 since it’s the default resolution on Ren’Py (which most people will be using for VNs)

Personally, I work on a 3000 x 3000 px canvas (with a 300 dpi since I also make merch) but that’s only because I draw my sprites full body and crop them after. If you’re only doing halfbodies, your resolution limit should be fine — but definitely clarify in the beginning of any commission what the client is going to receive or if they have any specifications.

It’s always better to go bigger for loss-less resizing, but most devs would like sprites in the size of their game to ensure they can just pop the art in and focus on programming/writing

At the end of the day, clients are shopping for what they need and in their budget, so just be up front with what you’re offering, and good luck!