I honestly think that solo BedWars with only 4 players feels a bit like a duel. I agree with you, having bigger maps for solo mode could make the games more entertaining and extend their duration. It would definitely add more variety to the gameplay!
Instead of making all solo BedWars maps 8 players, maybe some maps could have 8, others 6, or even 10 players. This way, the games would feel more diverse.