Certified GKH Gardening Expert
Your plant may grow up to five feet. Here are some articles that you may enjoy: https://www.gardeningknowhow.com/houseplants/crown-of-thorns
There are many hybrids which often affect ultimate size but the species crown of thorns grows to 2-3 feet. It can be pruned in early spring to keep it smaller and to help it branch out. If you have the plant tag, look up the specific hybrid. If you don't have a tag but remember where you bought it, search that website for specific information. There is a "red crown of thorns" which is a different species so be careful when looking.
Pruning guide: https://www.gardeningknowhow.com/houseplants/crown-of-thorns/cutting-back-crown-of-thorns.htm