Tent size will not matter much if the inner tent is mostly netting, which is the case in most/all ultra light tents. Even in the smallest tent, if the inner tent walls are netting, all the warm air you produce will be cooled down too quickly for it to make any noticeable difference. There is no thermal insulation whatsoever - there is the air inside the tent and there is the air outside, with only a thin sheet of plastic (fly sheet) that is dividing them. However, if your inner tent is netting only high(er) up near the ceiling, and if you properly tighten the guy lines so the fly sheet does not flap and is always and everywhere pulled away from the inner tent, and also you close the vents so the "warm" air does not easily escape through them, then there will be a layer of air between the fly sheet and inner tent which is not as cold as the one outside the tent and not as warm as the one inside the inner tent, and that layer of air will be a quite decent insulator (relatively speaking).
On my tours I almost always carry a 2-person tent that definitely isn't light (2.8kg, 6lb), whose inner tent is 1.5m wide (5ft) and the netting starts in the upper half, maybe several inches from the top (can't remember exactly). I spent relatively cold nights inside that tent (couple degrees below freezing) and inside was as comfortable as one can reasonably expect it to be - still cold but certainly much, much warmer than outside. A smaller tent would have been even warmer, but I like comfort and to have all my things inside, so the extra weight is the price I gladly pay every time.