Procedure cache (plan cache) size
You can't control plan cache size manually. The following Connect item describes both the fact that they won't be adding a knob any time soon and also that Resource Governor might be marginally useful in a few narrow use cases:

http://connect.microsoft.com/SQLServer/feedback/details/293188/amount-of-ram-for-procedure-cache-should-be-configurable

However, as I've alluded to in the comments, there are other ways you can make the plan cache more effective (e.g. use the "optimize for ad hoc workloads" setting). Kimberly explains this in greater detail, but essentially, this setting places much smaller stubs in the cache until the same plan is used more than once, and only then does it cache the whole thing. Under the default configuration, and especially in cases where a lot of dynamically assembled SQL is in place, single-use plans often waste a lot of space in the cache. If you reduce the amount of wasted space these occupy, you leave more room for plans that are used more than once, and those are precisely the ones you want to keep around anyway.