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A couple of ways to do this. Install and load the theme. Right click on the logo image and load it and see what size it is and where it is pulling from. Then make a logo that size and replace yours with it.
Or, open the theme file itself and see if it specifies the size of table that holds it. Or examine the code it produces by loading the theme and doing a view source in your browser and see if it specifies it there.
For instance, this XOOPS site specifies a 150x80 img in the HTML (funny thing is that the graphic is actually 148x80 so that effectively stretches it just a little). Otherwise, the table doesn't specify (nor does the stylesheets), so it will grow (or shrink) as the table grows if this one were replaced with a different size.
I haven't downloaded the subblack, although I did try the subgreen at one point. If that doesn't help let me know and I'll look it up and give you exact specifics.