@madcow: you can see the dimensions of a selected brush in the lower right of the Hammer interface. Alternately, while clipping, you can press o to see the dimensions of the clipping sides and such. Maybe that's enough for you?
Either way, those brushes shouldn't give r_speed trouble. As long as the textures on them have the exact same properties, they're combined into one face (though 240 texel splitting still applies, it won't give any different result than one brush would).