@tomalak
Based on this forum, it would appear that 3/7 (~43%) of <500 ladder maps are 5x5. However, you can check which maps are in your ladder map pool (their sizes are labelled) with more certainty by clicking the maplist button in the FAF client that I circled below:
Also, btw, there are a lot of other people who don't like to play on the types of maps often put in the ladder pools. A lot of those people usually play custom games at present, so you might want to try playing more of those.
Also, fwiw, I am running for PC (an elected position in FAF) in this election with a goal of adding options to the matchmaker, including having multiple map pools that players can choose from (so they only queue for the map pool(s) that they want to play with). If/when I am elected and matchmaker options like that are implemented, you should then be able to queue for map pool(s) that have lower percentages of 5x5 maps, as well as other variations.