@damiendelo
Father, one dimension is no more important than the other. Once your plumeria is rooted and growing leaves, it will fill that 4x6" pot very quickly with roots. I just up-potted one of mine that was in a 8"Wx10"D into a 12"Wx12"D pot. It had only been in that smaller pot for 3 weeks and that smaller pot was simply filled with roots. Plumeria grow mighty fast, given adequate room for those roots to grow.
Now the difference between the way you are growing your plants, inside, with artificial lighting and the way I grow mine, outside, in pretty much full sun, is radically different. I would think that your plants will grow much more slowly than mine and you won't have to up-pot nearly as frequently.
How do you tell when you need to up-pot? IF the plant is really well leafed, and is in a questionable size pot, I simply place the pot on its side and while slowly rolling it, I thump the side with the heal of my hand. That will loosen the root-ball and you should be able to take the base of the plant in your hand and just lift the plant/root-ball out of the pot. If its filling the pot, up-pot. If not, just keep it in the same pot.
I hope this is some help to you.
