In my experience it depends on whether electricity is available or not for a heat mat as in my experience a smaller greenhouse WITH a heatmat is better than a huge greenhouse without for getting things started early.
I used pallet collars on the ground to grow in all year round, and put up shelf brackets on one side of the greenhouse so that I could use decking boards to make shelves in the spring and autumn and take them down to string tomatoes up in the summer. And to be honest, I've harvested tomatoes up til Christmas Eve in the UK midlands before so sometimes some can't go back up til all the tomatoes are gone.
As tomatoes and chilli peppers came out each autumn, I put in rows of overwintering onions, garlics round the edges, sowed carrots, spinach, winter lettuce, kohl rabi, spring onions, claytonia, celtuce, coriander, parsley and transplanted the odd cabbage and kale if I had any already growing. I've put one photo of one bed filled with a mix of these below.
The good thing with pallet collars is also that you can get good deep loose soil for carrots [esp if you put one on top of the other] and just put a pallet across the top to give you essentially a work surface which you can put seedling trays on for your early starts and a heat mat if you have power in there. You can also plant much closer together and concentrate on giving a boost to the nutrients over winter where you want the tomatoes and peppers next year. I used to empty the whole of my wormery contents onto the top of my tomato beds which gave them a huge boost for the coming season.
My greenhouse was 6ft x 8ft and I packed so much into it, it was in constant growth for about 10 years without fail. I moved it from my back courtyard to the allotment when the committee changed hands and allowed us to put up structures.
I've added a couple of photos of pallet collar beds of spinach in my old greenhouse, I used to leave enough space to sit and plant in the middle, with inbetween space to get to the back of each bed and every other square inch was filled with crops. I harvested or pulled something out and something else went straight in. I had two other of these beds on the other side and I would get a carrier bag of greens from this greenhouse every week all winter long, it was fantastic.
In the small 60x80cm bed with the onions and spinach, I'd get 6 tomatoes in as well as basil growing below. I'd grow indeterminates along the sides near the glass and determinates along the sides without windows. And a crop of carrots would have been sown in pinches in between the spinaches.
I'd also put potatoes into pots which would go in any gaps and get an early start before the potatoes outside.
After my greenhouse went to the allotment I bought a small side of the summer house greenhouse, which had my heat mat in and in there I started all the seedlings for my garden, my allotment and also the numerous schools and gardens I was working in. I have put a photo of it but it was tiny but mighty.
If you can afford a smaller one earlier to the larger one, I'd do that and just start using it and make it your own. Everyone uses them differently so what I would do is nothing like what anyone else would do.