A maxi dress is just a Long Dress, not a short dress.
They can have straps, no straps, halter neck & so on. there are many types & styles available, that's just the name given to describe them.
Some are dressy. Some are not. If you are looking for something casual & comfy places like Target & Jay Jay's have them in the millions!!!
You can wear them with flats or heels, depends what suits. I've found being pg & having to deal with 35plus heat they are GREAT!!! Not at all too hot, just so comfy. I don't have to worry about anything restricting around my waist & I can show of my bump & boobs depending what style. I have 3!! and I'm about to purchase a chiffon maxi dress made by Wish for a Wedding I have next weekend.
Yes they are not much fun if they are too long LOL.