If it's a Goat Willow it'll have slightly asymmetric points to the leaves and they'll be hairy underneath.
Sizewise it'll get much bigger and eventually become a tree...might take a good few years though. If you like it and want a bit of height in your border you can always cut off the longer straggly lower branches and allow the main stem to grow. Would probably need support to keep it heading upright and regular pruning or it's gonna sprawl everywhere as it's doing at the moment.
Thanks. I may do this for the time being and see how it goes. I can't even remember planting it tbh, I think a bird may have dropped a seed or something.