Solid Boosters, sizes of components?

If I had to line out three things I wanted from SR2, it would be:

o Solid Boosters
o An easy way to directly set sizes and ISP
o Resize the main capsule

Maybe there are ways to do this that are simply not clear to me, but what I'd love to do is use SR2 to cobble together quick versions of the many small boosters that are now entering the market. I'm less interested in getting ships to land on distant planets, than I am of making decent versions of the Electron, Vector, etc.

Thanks.