Actually, I find the opposite is true. As you pointed out, the chemical reaction is exothermic and so the heat produced speeds up the curing process. If you use a big container, the heat tends to be trapped within and you could get into a thermal runaway situation. I won't tell you how many smoking canisters I've had to haul out to the patio in a rush while holding my breath (the fume is toxic).

Once the epoxy is spread onto the work surface, the larger exposed surface areas help to dissipate heat.

I find 35mm film canisters to be ideal for the West System Quart(resin)/Pint(hardner) pumps. I particularly like the transluscent Fuji canisters as you can see how much you have in them (I like Fuji film better anyways).

25-30 minutes is plenty for fiberglassing. The key is to mix small batches at a time so you are not rushed to use up a buttload of it. West does make a 45min hardner too. It works well too, but I find it takes quite a bit longer to reach full cure strength.

If you haven't already, buy a box of rubber examination gloves. HarborFreight sells them cheap so be kind to your fingers. Epoxy is toxic until cured. FG'ing is a messy business so don't do it over hardwood floors or brand new carpet. If the pieces you're FG'ing are small and/or require handling, build simple jigs or handles to hold them so that both your hands are free to handle the FG and resin. Wear old cloths or an apron.

Lastly, you don't have to use FG. When I lived in CA, I knew a guy that uses women's nylon hose to cover parts of his plane. Don't laugh. It is strong, extremely lightweight, and goes around curves no FG cloth can. I've experimented with some polyester fabric myself and they work reasonably well. The trick is to find the right weave - loose enough to allow the matrix (epoxy) to saturate the voids, but tight enough to offer the tensile strength required.