It's preferable to get at least 2.5 gallons because than your Betta will thrive and start swimming more often with more energy and the water changing scedule will be easier. If you got a 5 gallon tank you could cycle it and never have to do a 100% waterchange.
So long as you keep up the water changes, 1 gallon is okay. The bigger the better!
Lol I get it, Simon. Grumpy Betta.
Some more advice... By condition your water (as you probably know... but might not) They mean putting in a chemical such as aquasafe or prime that removes chlorine, heavy metals, ammonia, etc.
Be careful to only feed 3 or 4 pellets a day if you feed pellets. Unless they are huge pellets, lol. A Bettas stomach is the size of their eye, so it's really easy to overfeed them.
And of course you'll need an adjustable heater and keep the temp around 76 - 80 degrees preferably.
That's all I can think of for advice right now. Hope I helped some.