Size of header graphic in Product theme

Hi guys... I'm still having problems with Product, but I've tried several other themes (from here and from StudioPress) in the meantime and I think if I can just get product to work the way I need it to that it's my best bet.

Can someone just tell me how to find out the size of the header graphic I can use for this theme? I'd prefer to use the option for 'custom large image logo' and I know now that it can only be about the size of the word 'product' there at the top of the theme but I need to give specifics to the graphic designer and I can't find the specific size that it can/needs to be.

Thanks again!