Pot / kettle sizing

So long as you have the additional up front money to put down on larger equipment, there should not too much of an issue. Most things should scale down well enough. An oversized fermentor would be fine for primaries, since the yeast will be throwing out CO2 to fill the head space. Boil kettle shouldn't make too much of a difference, although depending on the diameter, you might get more boil off.

The main thing to keep in mind is when you mash in the larger mash tun, you'll have more dead air above the mash, more surface in the tun to initially heat up, and more surface on the outside to lose heat to. So, you'll want to try to compensate for that when you do your first batch.

The main thing you need to consider is the boil-kettle in terms of size, because of boil off. For example, you want a 9 or 10+ gallon boil kettle for 5 gallon all-grain batches if you're doing a full boil, because you will have boil-off and generally want to start with around 7 gallons of water, and will want some room in the pot above the water line. 7 gallons of water is fine in a 15 gallon pot also. It would be awkward to have say 3 gallons in a 15 gallon pot, but for boiling you should be fine with the largest size you'll think you'll want. For the other kettles (HLT), you don't need to worry about boil-off, so they don't need to be too much bigger than the batch size (1-2 gallons larger to give you some space and avoid spillage). They can be a bit smaller. People tend to do them all the same size probably more for aesthetics and convenience than necessity.