Radiant "In Floor" heat question

The size and spacing of the PEX is dependent on the required heat output which is dependent on your location, system size and operating temperature, slab construction, etc. You've basically asked what size of pants you need. We can't know that.

The manufacturer/distributor will size and layout the system for you for free. There's no reason for you to reinvent the wheel on this.

You can get that information by checking out the basics of radiant flooring from the manufacturer. What you need to know first is how much insulation you'll have in the floor, temperature of your region, temperature you want to maintain, etc.

In most cases, heating the slab is not a good idea in a garage. It is very slow to react both heating up and then cooling down so if you are using hte garage for a shop on weekends, it will cost a fortune to start heating it on Wednesday night to be comfy on Saturday morning. It may be OK if it is an every day use though. If you want to just keep your cars warm, well, that would be dumb and a terrible waste and probably rust them out when the road salt heats up.

You may do far better and save a bundle of money using infrared heaters mounted above.