The more room your plant has to grow, the bigger it will be in the end. It depends really how big do you want your baby to be? I think 3 gal is a pretty decent size tho
3 gallon is a great size for finishing out flowering in but some use 5 gallons when they have the space to use it. I start seeds in solo cups, move to 2 liters and then to 3 gallons for flowering. I believe if you give the plant a fresh pot to grow out more roots during that 2 week stretch it does help with the overall health and vigor of the plant as well as provide more yield as an end result. Just my 2 cents. Take it or leave it.