every sprite on a spritesheet has to be of the same size - but you can use different spritesheets for different sizes.
First you have to decide if a spritesheet should contain one or eight sprites (in 4x2 segments, see default for examples.
A single sprite has to be flagged with a $ in the filename, and it has to contain 3x4 single pictures. In the case of a 21x33 sprite, that would result in a spritesheet picture of 63x132 pixels.
A eight-sprite-sheet has a regular name and has to contain 4x2 full sprites, which means 12x8 single pictures. In the case of a 21x33 sprite, that would result in a spritesheet picture of the dimensions 252x264 pixels.