Modify mainmenu width to allow search to be inline

Hello,
I'm modifying my layout and module positions to match the visual design of the page.

What I need is the search box module to be to the right of the mainmenu module. However I cannot find where to specify the widths of these blocks and they are both currently set to 12. I need them to be a 10 / 2 split and be inline like other block. Please see screen shot of what I currently have.

I have copied the block code from the header.php file and pasted into the mainnav.php block but cannot change the block widths.

How can I achieve this?

Thanks

Lee