Hi Galla, and welcome!
16x16 is a really small character sprite for the UPE, unless you intend for the character to be appear small and surrounded by really large objects. If I were you, I would double the size to 32x32.
If you want to change the character mask in the Ultimate Platformer Engine, it will take a moderate amount of time and a moderate level of skill with Game Maker. There are multiple masks that are used for the character (Sprites > Course > Character > Character Masks), so you will need to adjust each one.
I don't know exactly what is causing your issues, but the most common reasons for characters getting stuck in walls is because of a poorly-made mask. You want the origin to be exactly in the center top of the sprite, with an even number of pixels to the left and the right of the center point. Even one pixel off and you can have problems. (The exceptions are mask_char_attack and mask_char_melee. If you won't plan to have the character using attack moves, then you don't need to worry about those)
Several of the mask sprites use custom collision masks in the sprite settings, so make sure that those collision settings are still correct after you resize the sprite.
Good luck!
-TingThing