This is Oreaster reticulatus. Size is normal.
I strongly discourage collecting stars without research - it is quite predatory.
And moreso because I don't recommend people releasing things back into the wild that are now possibly exposed to, for example, Indo Pacific live rock. May not be the case here but definitely please reconsider taking things. It can be a legal issue, but also just a case of getting something you may not want in the future.
This is also more of a open "seagrass" seastar and not so much a reef seastar. So the tank is not quite the right "habitat" in that regard either.