maybe no bigger than your spouse can carry into the greenhouse. ;-)
i don't grow them in containers but the article below is by a man who recommends container culture.
his article also details potting mix, fertilizing, etc.
i think you might find it helpful.
just my personal opinion but, assuming mailorder plants will be less than a foot tall, i would probably use something in the range of 5 gal., which should be sufficient for several years.

Here is a link that might be useful: michelia culture

Best bet right now would be something about 5-8cm larger in diameter than what they're in now -- too big a pot around a small rootball, and you risk wet soil and root rot, which michelias are prone to (that's how I lost mine).

Also, these are big shrubs, and don't like too much pruning, so you may have trouble keeping the champaca at 1.5m; the alba and figo are smaller growing and will stay smaller longer.

Enjoy the fragrance!

Jim

Can I ask where you found the michelia x alba?

I ended up buying them from a nursery in France. Quite expensive but I really wanted them. They arrived last week. I'm really happy about my Michelia Alba - it's quite a tall plant (over a metre in height) and already has four buds on it.

As for finding them locally, it is almost impossible to source Michelia Figos in the UK now (every nursery I contacted was either out of stock or their stock had been destroyed by frosts) let alone the bigger, rarer Michelias.

Have you tried the figo hybrids (M. x foggii)? I am not sure if they have a better fragrance or not, but I believe its a good alternative if you can't find the species version. I came across a UK website selling those a few times in the past.

I spoke too soon! Three of my flower buds on the Michelia Alba have dropped! One had even turned white and looked on the verge of opening!

Why do flower buds drop? The plant looks reasonably healthy otherwise.

Can anyone help?

Which nursery was that?

From my experience with a figo, the flowers lasted a couple of days and then dropped off.

I think sudden changes in moisture/temperature aren't brilliant for it, it's probably best off in a conservatory/greenhouse.

Sorry I've taken so long to respond. The ursery I purchased the plants from is Hodnik Plants and they can be found at:

www.hodnik.com

If you send an email to Andre there and let him know that Mr Mohindra referred you, he might be able to chuck in a freebie perhaps! In fact if anyone here does want to order from him, mention my name if you can. He also stocks the Ylang Ylang plants and vine, lots of tropical Jasmines, Citrus and Gardenias and a fair few Michelias too.