Simple question. Thanks

You have to edit the invoice and increase the size of the logo element/box.

To do this go to the menu Design -> New Invoice. Then walk through the wizard. In the section where it asks "Start with a blank layout or report" or "Use an existing layout", select the latter and the the template you want to modify.

Once you have the template infront of you go to the item that says logo, select it and then drag the corners increase the size.

Save it and then try it out.