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Pipe dope on the plastic/nylon gaskets, tighten hand-tight, and where plastic meets plastic. No leaks.

The longer the loop, the more pump power you need. The 5/8 could be any of those lengths. If you need to go long distances from the manifold, 3/4" could be an option.

Keep the glycol below 50% to help the pump sizing.

A snowmelt sizing program would spell out all the size, pump requirement and supply temperature required.

All the sizing depends on how quickly you want to melt the snow.

The Uponor design manual takes you through all the steps for the design.

https://www.uponor.com/getmedia/50a684b6-091d-4044-b8f7-c06f7705668c/snow-and-ice-melting-design-and-installation-manual.pdf?sitename=USA

There are a lot of things to consider.

Snowmelt most often operates between 15-35 deg. This is the temp range in which snow falls.

What % age of snow times and clearance do you want melting to occur. Class 1-2-or 3

There is very little difference in heat delivery between tube size 1/2" to 5/8" or 5/8" to 3'4" foot for foot. Heat delivered (BTUs) is dependent on Supply Water Temp and flow. Pex size has more to do with pumping power and tube length. Are you using Pex A-B-C? Use Pex A with an O2 barrier. I use 3/4" tube for increased flow and low pumping power.

You are going to use Anti-freeze, what % age? Anti-freeze increases the need for more pumping power at a given flow.

How is it to be installed? Sand Bed, Concrete, Asphalt, etc. Pavers going to be the finish?

How is the Pex laid out? Serpentine, Counter-flow. I always use a Counter-flow pattern. What about Pex spacing? Too long a loop length—too long and you could run out heat energy before hot water reached the end of the loop. 250' , I would think is the max.

What are you going to use (insulation) to prevent back and side heat loss?

Is it going to be an automatic of manual sys?

As with anything have a proper heatloss. done.

Uponor's C-dam manual has all that included. Shorter lengths are better.

The contractor should have all that done.