When you say learn on a small cc dirt bike do you mean that it really is easier to learn on a less powerful bike OR because of the lighter weight?
All the 4-strokes 250cc bikes seem to weigh the same as the EXC-F 350 so if it's really the weight, I would likely be better off with the 350.
Less powerful.
Heck if you were really aggressive and serious about becoming a master of dirt skills, I'd suggest a 150 two stroke like the KTM 150 XC-W or trials bike, even at your size and weight.
Takes more skill to ride than even a 250 4 stroke and forces you to learn a lot of the important dirt skills many folks never do as the bike (more power) lets them be lazy or makes up for less skill.
Frankly, I'm over 220lbs and would enjoy riding a KTM 150 XC-W. It would make me work and not forgive mistakes. But I could do everything I can do on my 300 smokers. Would I be as competitive racing on one, maybe, maybe not. But we are talking about gaining and mastering skills.
That said, the 250F's (f = 4 stroke) seem to be the consensus bikes to learn on for adults. I've seen more than a couple of adult learners who are accomplished street, track, even road racers struggle learning dirt skills.
Talk to Brian Garrahan about it when you take the class. I'd trust his opinion over mine or anyone posting randomly on the internet. :thumbup
If the goal really is to race, and race big bikes like a Rally bike, you will want to master dirt skills. You will learn and master those skills on smaller cc less powerful bikes....
I'm not claiming to be an "A" level rider, but have been riding for decades. This is just my experience and opinion, others will vary.
Edit: Remember, we aren't talking about forever bikes. But bikes to gain advanced skills fast.