I buy my tomato starts from my local nursery that grows their own....about 30 different varieties. Because we have such a short tomato growing season, I like that that they are already gallon sized. And I pot them up into 20 or 25 gallon sized containers immediately. The root systems develop very rapidly at that stage of development and there is no issue about what one might think is an over-sized pot.

Now there are many indeterminate types of tomatoes and if you didn’t know some need more space than others so I think you should transplant it to a 10 gallon max now it depends on your growing season but in general the reason why the 10 gallon is max is because when you transplant a plant that usually does not become that big (depends on the type of tomato) when you water It the water rushed to the side of the container then the plant will not be able to take up much water which means not much nutrients for able to absorb nutrients then the plant dies so try if the plant outgrows the 5 gallon then try a 7 gallon of it out that then try a 10 gallon

Regardless of variety, ALL indeterminate tomatoes can get BIG! 6-8' tall is typical but they can get as tall as 12'. All that top growth is supported by a very large and fast growing root system. And repotting/upsizing the container for a tomato plant once it starts flowering and fruiting is a very risky proposition. Plant now in the size you think you will need....and my suggestion is go as large as you can.

I've been growing tomatoes in containers with this philosophy for more than 25 years and have not encountered any problems.

Is it less risky if I upsize the container before the tomato plant starts flowering? I just bought the tomato seedlings so they're still very small.

I've grown many indeterminate tomatoes in 5-8 gallon pots (12-14" diameter). Some of my pots do have cracks in the bottom (oopsies with tomato cages/stakes being added too firmly), and then I have a couple more like 10 gallon planters. They need lots of staking and have to be monitored for water, but they produce quite well. The Shimmer plant we got this year is the first one we've been worried about really taking over the smaller pots; its going to go into a pot we deem to be enormous, it's probably about 15 gallons, which means it can't move at all once it's filled, which is why the pot was rejected for most purposes. Shimmer is expected to be 8'+ tall, most of our others are expected more in the 5-6' range. They'll sprawl, but that's what stakes and cages are for.

As for transplanting large plants, I find it does get harder the larger they get. Flowering doesn't make a huge difference (though I would want to get them potted up before fruits are more than a nub). The bigger issue is maneuvering the plant and getting the cages or ladders on ... though It might depend on what you're using for stakes and ladders.

If they're still babies and not completely pot bound, you've probably got time before they need to be in their final pots.

Make sure they're hardened off before they go outside full time ... sometimes nursery plants will come hardened off, sometimes not. You want to make sure they're used to sun and wind. A bit of light shade to start isn't a bad idea to start even if they are hardened off, in case your sunny space has more intense sun than they were hardened off in and to give them a chance to settle into their new space and/or pots.

For reference, I've been growing tomatoes, and various other fruits and vegetables, in containers for over 15 years ... not an expert, but not new to it either.

I'd transplant your seedlings to the container you're going to grow them in. I grow my tomatoes from seed in 36 cell trays and then they go in the ground. If you want to sell the seedlings you need seedlings as large as possible to get the maximum price. If your growing the plants for the tomatoes it doesn't make much of a difference what the size is.