It's not tank size, it's aquarist experience. Many breeders of show bettas raise & keep show winners in 1/2 to 1 gallon jars, no filter, heated room. That comes with experience.
Best bang for your buck in the US is a 10 gallon tank, dollar per gallon it's the cheapest, second is a 55 gallon. 2.5 & 5 gallon glass tanks cost more than a 10 gallon, due to the huge volume of 10 gallon tanks being made. It's common for used 10 gallon tanks to be sold at auctions for a dollar or two.
That being the case, for the new fishkeeper I always suggest a 10 gallon. I'm cheap as dirt, and I feel everyone should enjoy this hobby as economically as possible. The hardware associated with a 10 gallon is going to be nearly identical to that used in a smaller tank, the glass box itself is cheaper, and offers a few more possibilities.