Our yard is about 7000sq feet, and we pay to have it done as I got tired of fighting with wet leaves
50+ bags/year just from raking, nevermind mowing
Anyway ...
I ended up with a crappy Black and Decker 4HP electric when we bought a home
I had to lug around a very expensive extension cord, and the mower couldn't handle tall or wet grass, or thick piles of leaves
It sits in storage unused for years
I had a 3.5HP Briggs and Stratton and it handled things much better, but couldn't handle large dumps of leaves very well after a big wind, and was a huge chore if it was wet. But, it got the job done
And this is with a new blade, not used or hand sharpened
I'd say go gas and the bigger the better if you don't want to fight with it
A 4.5HP would be ideal, but I see many are rated in CCs now, not HP
And if you get a larger blade, say 21", then a bigger motor is a must or it will struggle