The default Array does not have a size as part of its type. There's a package StaticArrays which does this, typically faster below about 100 elements. But this isn't useful for catching mistakes before you run it, obviously.

Plotting is indeed slower than ideal, have not used Gadfly but Plots is more like 15s after restarting, then 10ms each time after. GR is faster, 5s or so the first.