A 24 x 45 sounds like a good place to start to me. enough room for say 20 bikes, 60 feet of work bench/tool boxes, and almost 1 bay work space. Nice. Could be more, but garage doors and small entry doors take up space
Will you be considering a room type web in the trusses for upstairs storage? yeah , for the stuff that isn't motorcycle related.
Are you considering insulating the slab? Good. Radiant in the slab? doesn't hurt or cost much to put in before the pour, even if you just want to use a hot water heater set up to get the slab warm.
Insulate the walls and ceiling? It's a must... even if you don't sheetrock and make it all pretty ,at least you can get it warm and keep it warm on any winter's day. Spray foam insulation is the winner in this event, fiberglass isn't nearly as effective and it just gives mice a place to live. It costs a little more up front but it'll pay itself off quick, depending how much you use the space.
I like a 4 foot lower area of side walls in metal... no rot ,no painting.
I look forward to seeing your new shop to house all these bikes. Q, has the lady of the house thought that it may be less expensive if you could just sell one or two of them?
Mine finally did. I just said " Ha! I'm off to the Lumber yard!" Totally ignoring whatever silliness was about to be expelled from her lips. As a back up plan , in case she brings it up again, I have been working on a "5 mile stare" , and have almost mastered my hearing loss into damn near deaf mode. Between the 'WHAT" and rhyming key words into the beginnings of a ridiculous conversation.... she kind of gives up ... it's been kinda fun for me. And I must say , it works well with shopping lists conversations about the in laws... anything, it's not just about bikes anymore.
But anyways , I hope you get to build a new shop.