We broke ground on our new house in Eastern Ontario that will have SIP walls. Hopefully everything goes fairly smoothly and that we don't hit too many snags. While I am very excited, I am also very nervous and a tad stressed as all the framing and most of the finish work will be done by me and my brother. I have subs coming in for the standard tasks such as plumbing, electrical, HRV, etc. One of my concerns is the sizing of the HRV which so many have written is usually oversized for a SIP home. I am wondering how I can ensure the HRV won't be oversized. Ideas?

Sure. Very simple really . Just have an accurate Heat Loss Calculation performed.

And(not so simple really) make sure the HVAC contractor sizes the equipment to those findings.

Since the HVAC contractor will usually assure you that his heat loss is accuracte, specifically ask him to do a "Manual J" analysis. That is more involved but necessary in a very well insulated house.
Bob I