I'd strongly recco a full garage - stick-built, kit (DIY or set-up by provider) in your area with cold winters.
10 x 20 single bay or 20 x 20 2-bay - or + 2' each more, if you have that much room, or even add 5-7' to length for "shop area" in front of car (nose-in) if room.
9 x 18 min or 18 x 18 otherwise to still have extra room. Too small for 914/911 only will ultimately waste your money spent, if you or later buyers can't use it for other cars IMO.
Cost wise on stick-built or kit won't differ much 20x20 ~ 18x18, etc. for a few feet of materials, relative to overall cost, so don't short change yourself. Steel is an option too, but may not be as durable & isn't rust proof for long, unless a high end Butler building or similar.
In NH/Northeast - remember that a permitted & with utilities, enclosed & insulated garage will add value & sale-ability to your house (really in any area this holds true).
Tough Shed is another option in many areas, & other pick-&-they-set-up prefabbed garage/shed providers are good options.
I agree with you pick the size & interior & outlets/set-up inside, & let the wife pick the "look".
Make sure to do whatever is considered an acceptable all-weather driveway from street to it also (some areas are okay with gravel, others the market &/or city/county require paved).
Think of it as adding a garage, & preferably a 2-car one - rather than a big shed - for best ROI. That too can be on your 1st morgage or HELOC 2nd Mortgage, etc., whereas a "shed" generally will not pass muster at cost for most lenders.
Good Luck!
Tom
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