My horse is on a dry lot because that’s often all that is available in Southern California. Her lot is 160’ x 210’ according to Google Maps. I love the size because she can really move around, kick up her heels, and not have to make sharp turns – unless she wants to – if she is being goofy and thinks she’s still a race horse. She has always been with only one other horse in this space. I love having the space as the horses age because it keeps them moving and healthy. For a while I had her in a smaller area, by herself, which was 40’ x 95’. She definitely moved a lot less in that space.
OP, I think your dry lot size is perfect, especially since they will have access to a large pasture much of the time.
I will mention one thing which may be of interest. When I had my mares in the adjacent paddocks, they were aggressive with each other, especially at feeding times. I think the main reason was the location of the gates. The two gates were right next to each other along the front fence line. Both horses would move toward their respective gates, which meant moving toward each other, when a person approached one of the gates, and then there would be a lot of pinned ears, ugly faces, and butts turned toward each other for kicking as they each defended their space. All of that stopped when I put them in together, but I think it would have also been prevented if the gates were several horse lengths apart, say, at least 30’. So that is something to consider when you are deciding where to locate the gates to your paddocks; place them in the middle, not at the corner.
The barn owner had housed two of his mares in those paddocks a few years earlier, and it got so ugly that they got into a double-barreled kicking match and really injured each other.