Yes. "STOKE can be used in many different scenarios, such as optimizing code for performance or size". Note that this is the 4th sentence in the linked page. However the README hints that STOKE is unlikely to improve much over gcc or llvm optimizations such as -Os / -Oz for binary size.

I guess it depends on the optimization function? However, I suspect to avoid loops there will be loop unrolling and to avoid interprocedural analyses (which I believe is beyond the scope of Stoke) it may use inlining, which may increase the size of the code.

Have you seen upx? It might be what you want