Choosing a baptistry for your church is different than choosing church chairs or a pulpit. When choosing a baptistry, you will have to consider the size of the platform and any doors that you will have to go through with the baptistry. If you have a small platform, you will need a small baptistry. If you have small doors, that does not insinuate that you need a small baptistry. there is a baptistry that comes in a split version. A split baptistry comes in two pieces and when you get the two pieces where you want to put the baptistry you connect the two pieces together.
Here are some other things to consider also. If you plan to leave water in the baptistry, you will want to put a coat of sealant in the baptistry. A good baptistry heater is a great addition to have but not always thought of when purchasing a baptistry. Some churches need a portable baptistry to move from one area to another. There are baptistries that come with this like this or you can add your own casters to it.
It is always good to get a professional plumber to install the baptistry. There will need to be a water line coming into the baptistry. you will also have to have a way to drain the baptistry when you are done.
another thing to think about is if the minister wants to stand on the outside of the baptistry or be in the baptistry with the person being baptized. I have done it both ways prefer to use the dry method where the minister stands on the outside of the baptistry.
Some baptistries come with a window on one side so that the congregation can see the baptism. There is an option to have a seat in the baptistry; I would strongly suggest this. The person being baptized would sit on the seat and when they are baptized they would be leaned back and brought back up.