CSS Buttons and fixed width

[CzechBoy]

Here:

http://www.zen-cart.com/wiki/index.p..._-_CSS_Buttons

you can find this:

Style
For an unknown reason, Zen Cart™ appears to calculate the number of characters in a button's value and then generates a length in pixels based on the size of the value. Would it not be wiser to allow the button to choose its own size?
If you know why a width is applied to CSS Buttons please share, it would also be great to be able to disable this feature.

Does anyone know how to make fixed width of a button? I want to make background image (see atachment) but I dont know how to... another way would be to stretch the background image but I guess it is not possible either...