Well, it will certainly overwhelm that obelisk in no time!! Yes, montanas get big - very big!! This is a picture of one in my area I posted last year:

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Montanas bloom on the previous seasons' growth so they are considered "no prune" to maximize the flower potential. You can prune immediately after flowering to control size but I doubt you will be able to successfully keep it as small as you want.

My suggestion would be to remove the obelisk - too attractive to be buried in vine - and train the clem to grow on the chain link fence. It's vigorous enough to grow in both directions (left and right sides of the corner) and will make a very handsome covering to the fence. Save the obelisk for another rose or a smaller growing clematis.

Yikes ! 20 ft ! What a difference from what you are expecting I'm sure . I hope you find another one to use . Also I hope you are happy with your Eden ! It requires a ton of patience. I'm on year 4 And every year I swear I'm getting rid of it . It just dosnt bloom much for me . Mine is own root though. That may be why