A 40 will work fine for the training dives up through Adv Nitrox and Deco Procedures. At least with the TDI courses due to the limits on deco for training. That said if you have a super high SAC rate it could be cutting it close. I own a 40 and a 30 that I use interchangeably. Depends on the dive.
What I would advise you to do now, whether you own or get a 30, 40, or even 80 is to get it rigged correctly, and start carrying it on easy, shallow dives.
Practice putting the bottle on and taking it off in the water. You don't necessarily have to deploy it the first few times but just get used to having it.
See how it affects your ability to access other items and if it has any effect on your trim, and make any adjustments before you show up for class. The instructor is likely going to make some recommendations to your set up anyway but you'll get a head start.
Once you've carried it on a few dives get with a trusted buddy and have them stay within arms reach, ready to donate a reg if necessary, and practice deploying it and getting on the stage/deco reg. Until you get your advanced training the gas in the bottle should be the same as your back gas. You still need to go through the gas verification procedure.
This helped me out a great deal before venturing into tech training.
I dived with guys who gave me tips on setting the bottle up and practiced using it. When it came time to take my NAUI Intro to Tech and Helitrox class I had close to 50 dives in doubles carrying a 40 stage bottle.
It's also why I have students carry a slung bottle in the Advanced Open Water class. Along with it being a redundant air supply, it's good practice to get divers who at some point may decide to go the tech route used to slinging a bottle.
For Advance Nitrox and Deco Procedures I want to see students show up for class with a minimum of 25 dives in the configuration they are going to use, including stage/deco bottle, dry suit, doubles or sidemount, etc.
It makes the training go a lot easier if I don't have to show one student how to mount their deco bottle.