It depends on what you want to grow.

I have a 6x8 Harbor Freight greenhouse with twinwall polycarbonate and love it - wouldn't give it up for anything, except perhaps a larger greenhouse. I overwinter annuals, tender perennials and tropicals, keep fresh herbs, salad greens, cherry tomatoes and lemons growing throughout the winter, then in spring get an extra-early batch of sugar-snap peas and start seeds and cuttings for my flower and vegetable gardens. Houseplants take turns out there for "R&R", and that's where I start forced bulbs for winter color insie the house. It's also a delightful refuge of green and growing things during the cold months of winter. Yes, there's a chair out there, and while I'm the only gardener in the family, others have been found out there ... just "chillin'"

We're on a 50x150 lot, but I would imagine the results would be about the same as long as the space and sunlight are available. For winter, I do insulate the north wall with 1" foam board, wrap the g'house in a clear solar pool cover, and stock it with plenty of water jugs to help maintain an even temperature. Heat is from a 1500-watt convection heater (though my first choice would have been an oil-filled radiator type, if one had been available), with a small catalytic that attaches to a one-pound propane cylinder on hand as backup in case of a power failure. It does have an automatic (pneumatic, not electric) vent opener, which opens surprisingly often on winter days, and a small fan that runs 24/7. The HFGH runs about $300, the foundation, heater, fan, pool cover, thermo/hygrometer, bench and floor materials, etc. brought the total cost to about $800. I couldn't calculate the electric last year because the electric company completely messed up our readings/bills, but it wasn't horribly higher than previous years (especially in view of the enjoyment and fresh produce we got).

Just remember that the smaller the greenhouse, the more difficult it is to moderate temps - it will heat up, and cool off, more quickly than a larger structure.

A good way to save energy with a small greenhouse is to build it "Lean To" ie. backed against the house/ apartment a bit like a mini-conservatory.You will probably need to take the plants out in the summer, however, as they can get very hot. I found that an automatic vent was very useful.