Not ragging on you-- because good for you, thinking about a small house-- but 1800 s.f. sounds huge to me. We (family of 3) live in about 1200 s.f. house and it seems pretty big, except for the lack of a garage (which we converted into a rental apartment, so there's that). We are working on remodeling a different house that is maybe 1300 s.f. but has both a garage and a space for a rental apartment attached.
We'll see. We haven't decided on anything yet, and there wasn't much on display in the size range between 1200 (which seemed a bit small, if entirely doable) and 1800-2000 (which seemed a little bit large, but had nice kitchens and pantries, which we want). Hopefully this summer we'll start making more specific decisions about what we want, and can figure out the tradeoffs involved. I personally think 1500-1600 sq ft would be about ideal, but we'll see where we end up. There were definitely some wasteful uses of space in some of the homes we looked at (hallway zigs that were literally useless space but probably 80 sq ft used), and we want to minimize those space wastes. It's also easy to add a room to the end of most of the homes which could be used as a playroom or general utility space. I really don't think we'll end up beyond 2000 sq ft, and probably will settle in a good bit below that.
Some of the space is going to a nice kitchen. My wife cooks pretty much everything from scratch, and we're planning on a decent garden, so having a larger kitchen with a lot of space to work (also pantry space) is important to us.
However, this also doesn't count office space. I'm planning to retrofit a shipping container or two as office space/lab space, since I'll be working from home most of the time, and need places for some development hardware racks. I don't really count that in home square feet, though.
Just something else to think about-- I understand that manufactured homes don't hold their value as well as traditionally built homes, and don't tend to maintain their condition over time. The plumbing and fixtures tend to be less than dependable (maybe you can upgrade them at the beginning? I don't know-- I'm speaking from personal experience as a former tenant of what is likely the lower end). You might want to consider the life-cycle costs of buying a manufactured home vs. constructed, including the re-sale value at the end of your use.
"I don't care." :) We don't intend to resell it, and if we get nothing out of it after 30 years, I'm fine with that (we wouldn't be paying that much for it to begin with). We have an opportunity to move onto a corner of my wife's parent's property (17 acres in rural Idaho), and pretty much just pay for the structure and some upgrades (the septic system is 40+ years old, so that will need some work). We'll have a lot more land for kids to run around on, grandparents up the hill, and other family in the immediate area, vs where we are now with none of that. It's much better suited to our desired lifestyle than suburbs.
Also, newer manufactured homes are rather nice. 2x6 construction, heavily insulated, drywall, nice counters if you care, etc. They're not a 1970s trailer that most people think of, and we'd be putting it on a proper foundation (possibly with a basement - haven't decided on that yet, depends on cost since we'd be putting it on the side of a hill). Manufactured homes are rather common out in Idaho, and on most of them, you really can't tell the difference between it and a stickbuilt house without paying close attention. They do tend to be more square, missing all sorts of weird angles that leak heat, though.
As far as maintaining them in decent condition, you can certainly let one fall apart, but I've yet to be convinced that they will fall apart no matter what you do. If it does start having problems in 20-30 years, well... kids will be in college, and we can build something out of shipping containers for my wife & I. :)
Manufactured homes are fine if you can fix them up. And of course, they come in a variety of finishes (my BIL used to make them, and I have family and friends who live in them).
So my sister and her husband's double-wide is probably 20+ years old now. Built on a basement foundation with a deck and all. And over the years, they've replaced the walls with drywall and replaced some of the plumbing, etc. It's pretty much a real house by now.
The new ones can come from the factory with drywall, and once they put it together and tape it up, you really can't tell the difference. We plan to go with a foundation, and probably a deck at some point. I'm also planning to play with solar heating and cooling, and a bunch of other fun low tech solutions, so having something modifiable is useful to me. DIY solar radiant heating, perhaps? :)
It's an unusual approach, but we've thought it through and are convinced it's the best option for how we want to live life and raise our kids (close to family, in a rural area, somewhat self sufficient, free to travel and explore).