The 35k is the file size. If you look in Windows Explorer or in the properties of any file, it will show you the file size. Your siggy is 7142 bytes which equals roughly 7k. Divide the number of bytes by 1000 to arrive at the kb (more or less).

Height/width:
125 = the height
600 = the width

So...
- don't make it wider than 125 pixels in height or 600 pixels in width.

and/or

- don't use a graphic which has a file size of bigger than 35k

Does that make sense?