I dont play ladder much for precisely this reason. Also almost every time i see a 1v1 hosted as a custom game its for a small map (which arent catered for well by matchmaker), suggesting i may not be the only one.
Basically maps larger than a 5x5 for me are mostly too much energy or work instead of fun.
What about having a map preference system coupled with a map veto system and a smaller pool? I.e. say you have 3 or 4 5km, 10km and 20+km maps each in the pool, with 3 or 4 vetoes and 3 preferences. You get matched with someone ignoring this (so no change from normal) then when it looks for a map it looks for common preferences and either chooses these or makes them far more likely to be chosen, while vetoes cant be chosen.
Depending if you do 3 or 4 you allow people to remove an entire type of map size without impacting on how easy it is to get a match