Need sidebar 300 px

Just need my sidebar to be 300px at max width to display the recommended medium rectangle ad size. If I could even make the max page width the extra 50px that would work great for me, pushing the sidebar below content at smaller widths.

I know it was talked about here but, that solution breaks my responsiveness which totally sucks because more than 90% of my views are mobile devices. I’ve been messing with it all morning and can’t seem to find another solution