No I'm not very good with BASIC language. I'm guessing you're new? We have a really good community called "SmileBasicsource" look it up. People are often more than willing to help you out! :)
You can specify a custom width and height for a sprite with SPDEF. You'd specify the definition number, the starting coordinates (the top left corner) of the sprite on the graphics page and the width and height. Like this...
SPDEF <DefNum>,<OriginX>,<OriginY>,<Width>,<Height>
OriginX and OriginY would be the sprite's top-left corner's coordinates within the sprite sheet. For example, in the default sprite sheet, the first one would be on (0,0), the second one on (16,0), the third on (32,0), etc.