Most if not all the NPC's internally are 32x32 for each frame. Some exceptions are large NPC's, such as large Pokemon, that use 64x64 for each frame. If you check out the Spriter's Resource for HGSS, you can see some human NPC's can get pretty wide. Claire's side frames and Whitney's front-facing frame for example almost reach the edges of their 32x32 boundary.
Personally, I would recommend using the space necessary for your sprite and then chop it down if it feels too large. starting with a small restriction like 16px wide leads to pixels hugging the edge too often.