if you are referring to breyer model horses then Traditionals are far more collectible, while the classics are meant more for playing with.
Banjo the Breyer Horse is a traditional and he is retired : (
Most Traditional saddles do not fit Classic Breyer horses. Although, Breyer made some Traditional horses like Newsworthy and the "My Favorite Horse, Appaloosa" that are more Classic sized but they do fit in the Traditional saddles. Breyer does make saddles for Classic Breyers but they aren't fancy like the Traditional ones are.
It varies a bit, but the average length of a Classics horse is about 5 1/2 inches.
First of all, the website you're buying it from should either say the inches the horse is, or the size (traditional, classic, stablemate etc.). Then, if you want the inches you can research the inches of the size (traditional, classic, stablemate etc.) it is. If the website doesn't say either format of size you should contact the seller and find out the size.
If by classic, you mean english, yes. what is most important is the horse's comfort
"Whoa" is the classic command to stop a horse.
the grey mare is the better horse.
There are several horse models, available at the related links below.
Breyer
ShopAnatomical.com has a good horse skeleton anatomy model.
The Epsom Oaks.
Slug horse