ILuvChubbyChix:
I don't know if there's a really good answer for this, since it really depends on how far you take it, and there's so many things to possibly consider. The best thing you can probably do is read about the experiences of others.
There's tiers and levels of fat that most folks never get to, for one thing.
Just as a forewarning though, some of these can potentially be quite expensive, so you'd need to prepare for that.
First Tier:
These are probably some of the first changes you might notice.
Food Costs
Perhaps obviously what the food costs, though this might not be as big of a deal depending on what your current eating and spending habits are. Of course, I don't know what kind of budget you're on. If you're already engaging in a lot of budget unfriendly activities, it'd be easier to adjust.
If you get same day food delivery - stop. It's so insanely expensive it's not even funny. You can get SO much more food from the grocery store, or even when you get takeout. I will never understand why anyone uses this service.
You shouldn't use grocery delivery either, but if you somehow must, try to maximize the size of the order to economize on delivery/shipping costs.
Fresh fruits and vegetables are also expensive, spoil very quickly, and lack calories. Vitamins are important, but multivitamin pills are very cheap.
Clothes:
How much this costs of course, depends on your fashion choices, preferences, and requirements. If you need to wear 3-piece suits for work, this is gonna suck, though this is less likely than once upon a time.
The first thing that would stop fitting are bottoms; trousers, jeans, skirts, etc. especially around the waistline area.
The next thing that will stop fitting are tops; shirts, blouses, etc. particularly around the neck area, followed by probably the gut area and depending on the item, the arm holes.
While less common than before, there are still some clothing brands you will have to give up on. This isn't a problem that just women may have, but men could have it too. I notice that Express won't sell bottoms larger than a 42 (44" waistline) for men, which is too bad since I think they make some decent quality stuff and if you catch the right sale, the pricing isn't too insane.
That's tier 1.
Second Tier:
This probably won't differ too much from tier 1, but other things start to be affected.
Depending on the shape of your foot, or how much that's thickening up, you might need larger shoes. Though, you're unlikely to change shoe size anywhere near as quickly as clothes size. This is why I didn't mention shoes in tier 1.
Jewelry, in the form of rings, may eventually need to be resized, or moved from say, the index finger to the pinky, etc. This process is not particularly cheap. When I had a ring resized, I had to pay about $90.
Some furniture will start to give you pause, but such furniture will be rare. I'm talking about small, thin plastic stools or smaller/thinner folding chairs. Traditional classroom desks with an integrated chair may be too snug, but I don't think those are even common in classrooms anymore, and you'd never find them anywhere else.
I could be wrong but I don't think most folks usually get too much past tier 2.
Third Tier:
This is probably the point where more folks might start to think "Wow I'm big" since this could require more expensive changes.
You might also be thinking about another bed/mattress, though this might happen in tier 2.
If you have a compact car, you're probably giving serious thought to something bigger. Depending on model, this could mean it costs a good bit more.
There's more furniture where you're worried it might break. You're thinking about a desk chair designed to hold more weight.
You can probably still sit in a booth at a restaurant, but it may not be comfortable so you prefer not to.
Commercial flights will be even more annoying, since coach seats are just too snug. Public transit is likely to have the same problem.
Fourth or Fifth Tier
There might be a tier where major dwelling space renovations may be required, but realistically, few individuals ever get this big.
I feel like this is all really solid advice and it definitely encompasses things that I wasn't thinking of. I'm a long way off of being too big for most furniture and my car, but it'd definitely be something to consider if I were to go all in on getting as big as possible. I know sometimes I fantasize about being 400-500 lbs, but realistically, if I gain, in the short term, I'd probably be looking at about 200lbs as a goal with 270lbs being the high end of that goal.