First try it out with regular 6.5 gallon buckets.
Once you like or prefer brewing smaller batches, and do enough of them, maybe look for 3.5 gallon buckets (for 2.5 gallon batches). Aim for a gallon of headspace.
Going from 5 to 6 gallon batches to 3 or 2.5 gallon ones, all things in a recipe/process scale fairly linearly, except for a few:
- Boil-off (evaporation) in the kettle will pretty much be the same, whether you boil 3 gallons or 6. You can always top up during or after the boil too, or in your fermenter, and make adjustments on the next batch.
- Your mash temps may drop more and faster during the hour mash, due to heat losses being similar, but with half the thermal mass inside to compensate. That's harder to overcome. Start at a few degrees higher and bulk up the insulation.
I often brew smaller (2.5-3 gallon) batches anymore, it gives me more variety. I can easily do 2 of those back to back.
Or brew a 6 gallon batch and split the wort after the boil for different whirlpool and dry hops.
Or split a batch over 2 or more fermenters using different yeasts or additives.