It should reach up to about 2 metres in the ground, less in a pot, but if its been in the ground for 10 years, it won't be that easy to move. It is, though, a Prune Group 3 clematis, which means it should be cut down to about 8 inches by mid February every year - if you haven't been doing this, then do that and see how it does next year. Further info on this clematis below

https://www.rhs.org.uk/Plants/80890/Clematis-Arabella-(I)/Details

So long as it gets enough sun, you can improve the soil by simply applying a couple of inches of composted materials a couple of times a year, when the soil is damp - either your own garden compost or composted animal manure from the garden centre.

I grow 'Arabella' through a variegated euonymus. She was planted 15 years ago when the shrub was already mature. She gets cut back hard in spring and given a good feed, and has always done well for me

No, the euonymus doesn't get pruned at all