I've never seen them offered for sale anywhere or even seen them growing in someones' garden. They would only overwinter in the far southern states where there is little frost. I can't imagine anyone else overwintering such a large plant indoors. In other words such a large, frost sensitive plant wouldn't be very practical to grow anywhere that got any amount of frost.

I would think The only place you would see such a thing is in a university or botanical gradens greenhouse.

I don't know what variety this might have been but my understanding of the OP's description is that the head gardener used a technique of overwintering pelargoniums which was common before the advent of easily seed raised varieties. It is still commonly used for perpetuating favourites cheaply. At the end of the summer the 'mother plant' is cut right back and stored undercover in a frost free place. If not already in a pot it is potted and kept on the dry side. In the early spring it is brought into growth by watering and, possibly, feeding. The new growth is used for cuttings which are grown on that summer. There is no need to overwinter the entire unpruned 3 foot plant, just the roots and cut back stems. Alternatively, the autumn trimmings are used as cuttings which are then overwintered under cover in pots but this requires more space for longer. The link explains other ways of overwintering pelargoniums.

Many older varieties would grow this big so we need more detail. You don't even tell us the colour so it's tough to id it!

Here is a link that might be useful: Overwintering pelargoniums

Sorry I'm remiss for not providing more detail.

flora_uk you described exactly what the gardeners at the university did! One year I even helped the head gardener prune back the old plants for storage. And after he selected the new cuttings he wanted I took a few home. The next year they grew into huge plants, unlike today's modern hybrids.

The flowers were a bright cherry red. The leaves were a medium green, without dark zones. The oldest stems took on a dark color with many nodes for sprouting if the plant was pruned back. The root system was substantially larger than modern geraniums.

He talked about having pink ones over on the south side of campus but I never saw those.

One of the most widely grown red pelargoniums was Paul Crampel. Maybe that was what you had at Uni. There is a picture at the link. You need to look down the page a bit. It's in the 5th row. Also don't get mixed up with a rose with the same name.

Here is a link that might be useful: Paul Crampel

"Old Fashioned".....what's old fashioned...a plant that somebody in his 60's, 70's planted when he was in his teens and twenties....
Plants grow up.....what was planted back when, is really the same plant planted now except it has been brought forward by up-to-date science.

To bring an annual pelargonium through winter is no challenge at all ...IF the gardener can provide the plant with what is necessary...a cool environment, darkness and the willingness to NOT water. And that's all that's necessary.

If the person cant provide those necessities, then he/she is invited to keep the plant going over and through winter as though it were a houseplant or, by cutting a healthy plant into pieces and saving them as tho a houseplant.

So....that's all it takes....there's no magic science to not carry an annual geranium into the next spring.