Usually you would use sizes that can be devided by 2, like 8,16,32,64 etc. At least for images (not sure about tilemaps). It is better to process for the gpu and compressions work better.
Are your sprites 15x15 too like your tiles? I can imagine Godot not being sure how to squash, for example, a 16x16 sprite into a 15x15 tile? Not sure.
In ProjectSettings->Display->Window did you set a stretch mode? For pixel art the option "viewport" is recommended and scale mode "integer".
Do you have a screenshot?