Nope,you don't have to stick to 3 gallon batches & all that. I do Partial Boil/Partial Mash biab myself. I mash 5-6 pounds of grains in 2 gallons of local spring water @ 152-155F,usually 152F for most beers. Maybe 155F for dark beers. Anyway,I put a cake cooling rack in the bottom of the BK/MT to keep the 5 gallon paint strainer bag of grains from burning on the bottom.
When I get the water up to 150F,I stir in the crushed grains till all dough balls are broken up & they're evenly wetted. When the temp comes back up to 152F,I have a winter hunting coat with two pot holders & wrap the covered kettle up in with a floating thermometer in it for the 1 hour mash.
I heat 1.5 gallons sparge water to 165-168F. After the mash is done,I pull the bag & place it in a SS collander on top of the BK/MT to drain. Then slowly pour the sparge water over the grains & let drain again. Pressing on the grain bag a little to get them drained out more.
This gives 3.5 gallons of wort for the boil. I do all hop additions with this,since it's 50% of the fermentables. boil off is usually 1/2 gallon. I add 3lbs of plain DME at flame out. Chill down to 70F,then strain into fermenter. I started using super moss,so you wanna get any excess gunk out. Then I top up to recipe volume with cold spring water out of the fridge (or garage in winter). Pitch yeast starter or rehydrated yeast & seal'er up. I've done 5 & 6 gallon batches this way.