I'm going to be building a home far up north near the Great Lakes. I was thinking of using solar heated water in an underground tank instead of a pond for the loop. Less loop more heat.

Would this work?

It could work if the solar heater produces enough heat and you have a backup for long cloudy periods. I wouldn't expect a tank to be good for more than a few days of heat (unlike a pond that can store for a season).

I was thinking of pumping up the water from the well and then heating that with solar. Lots of wind and water where I'm going to build.

Very often the coldest days up there are the clearest. High pressure system brings in the cold air but not the clouds. the cloudy days are usually the warmest.

So I figure I'd have the well water temps on the warmer days and the solar heating on the coldest days to augment the well water.

Does my reasoning make sense? Is it likely to work? Any ideas on how big a holding tank I would need for it to work?

Design would start with the heat loss of the home (manual J).
Do you need 5000gals of 50* water/day or 4000gals of 40* water/day.......?
Load would help determine that.
The problem with the storage tank idea is always how big. That of course is determined by temperature and requirement. Without significant solar contribution, you are really operating an open loop system. In that instance, it might be cheaper to cut out the middleman storage tank.
Good luck,
Joe

Your idea makes sense,  it might work.  The success or failure will be in the design.  What is heat load of your house?

Manual J load calc would be a good start.

Hourly Bin data may be needed to work out maximum daily load which will help inform tank size. That and an educated guess as to how many cloudy days the system will carry through.

Whether this works or not depends on WHERE you do it. If you're east of the lakes, forget it. WAY too much cloud cover in winter. I live in central michigan and we get very little sun in winter.