What size pressure tank with 1/2 pump

Tank size is not determined by pump size. Tank size determined by water usage. The larger the tank the less often the pump has to come on. Major factor in pump life is the number of times it has to start and stop. The less often the pump has to start the longer it last.

My thought is to install the largest tank you can fit in the space you have. Saves you pulling and replacing the pump later.