Fixing width of sidebar

My site has 2 sidebars, one primary one on the left and one on the right for ads. Both of these are relative. The left sidebar adjusts fine because it covers almost 1/4 of the site. However, the right sidebar becomes almost non-existent when I adjust the browser to not even half its size. This makes ads I have in the right sidebar become absolutely miniscule. What I want is for this right sidebar to remain fixed or at least have a minimum width.

One of the solutions I’ve tried is to create a new sidebar, but what happens when I add it all that happens is a huge black horizontal bar appears at the top of my site. Could I go about maybe editing the CSS of this sidebar to be on the right-side with a fixed width of say 250px? Would share the site here but it’s filled with porn.