Do I need to pre-size this or can I cut it to length myself?

You can cut it with a sharp blade or utility knife. Just make clean, straight cuts to avoid fraying the edges. If you're using barbed fittings, a little warm water on the inside diameter makes insertion easier.

Will silicone hose work for my coolant system?

Silicone handles coolant fine, but verify your engine bay temperatures don't exceed the hose's max rating. If your setup runs sustained high heat, confirm silicone is rated for your specific application before committing.

What clamps should I use with this?

Standard worm-gear hose clamps work with 3mm silicone, but stainless steel is preferred to avoid corrosion. Tighten firmly but don't over-cinch — silicone punctures easier than rubber if crushed.