Peninsula size. I had a whole area planned out for mine before I realized I got the smallest size. Also not being able to change the location of the secret beach.
I wish I knew to pick a map with resident service further back so I could make a grand entrance. I didn't realize the importance until I started to terraform. My airport is so close to the airport and I can't do very much! Although I don't want to reset because i've come so far. So I just made the best out of my situation and I am content.
I wish I’d go for a map without two south
river exits, having two narrow strips of land on bottom two corners is awkward (east/south river is best imo) and a wider peninsula to fit ables onto, I’ve made it work on my original map but its just a lot more trail and error with terraforming. Also to not waste time building crazy watefalls as soon as I unlock terraforming, you will end up finding the loud water annoying and knocking it down again
Here are some things I wish I knew... Airport color determines your Nook Miles Items, pier can be different sizes (would of tried for the longer one), Resident Services isn't in a set location and it varies wildly (same with Airport). I thought everyone's Airport was the same distance from their RS, haha. Islands have native flowers.