Size shouldn't matter as long as the FD cable clears. The Rd cable leaves at a tangent off the bottom of the shell, so it's never a problem. Even if the FD wire rubs the back of the BB shell between the chainstays, that too, is OK If you're concerned, a bit of protective tape will prevent any damage to the paint.

I tell you this to open up your options. There are nice steel guide available at reasonable prices, and they'll outlast your frame.