FOR ONE:
this depends on several factors, what is it that you are interested in trying to do; also what size flasks are you to use in the experimental procedures. Once you can answer those questions for your SELF then, it is my guess that u will be able to provide your SELF with the adequate info for what size stir/heat it is that you require...
My guess is that you want a standard size, as even if you wish to heat bigger flasks,' then the bottom base plate it really does not matter as you will most likly use an oil bath to provide stable heating, and a better distribution of heat'
For example any of these will be suffice: as you can see they vary in size'
In the end it all depends on what it is that you are trying to do' might i suggest that you work out the size of you flasks that, which you are going to use and then work out what size apparatus it is that you require.. As most heat/stir tell you how many litres they can heat.
all the best' If this does not help then FUCK knows'