The upload page lists the size limit as 300KB. You didn't say how big your current image is so I can't provide any assistance with what numbers you'll have to use to get to 300KB.
I normally resize in MS Paint if it's a simple resize. Open Paint > Image > Stretch/Skew and change the values in the Stretch section to something appropriate. Use the same numbers in each box. Make sure you Save As if working on new images. In fact, I normally create a copy of a new image and work from that. Once the image is saved, check the size.