You can always just cut cubes to the size needed. I do all the time. You should have them in an odour proof ziplock anyway as they stink out gear even when in the unopened packets. Just score the cube with a knife and then they will break along the scored line in your hands.
You will probably need larger cubes anyway when using them in the real world eg cooking in the bush, also depending on the application.
I’ve done quite a bit testing with a wide range of esbit burners and set ups both at home and in the bush for a number of meals (TD fissure, Jon Fong snow leopard/epicurean, BGET, coghlans, diy varieties) and authentic esbits (7, 14, , and 28’s), coghlans, and imported alternates,as well as different pots.
Esbits and esbit stoves are not linear and simple – they need experience to tune them to your particular need, as their performance and hence fuel needs varies greatly based on tablet size, the environment, and what style of heating you want them for. Stove design plays a part too, especially windscreen design.
I’m not convinced that there is enough material advantage between tablet brands when using them in a real life settings in the bush to warrant any worrying over them Esp when, as normal, there is a breeze or wind. I have measured a very slight advantage to esbits over competitors in indoor conditions. But in the bush you can lose up to 25-50% Performance over what you see in a sheltered room indoors. Eg with a highly optimised kit I find I typically need about 1.5 14g cubes to bring 2cups/500ml to a rolling boil at 8mins in windy bush settings, whereas I can get a boil with one cube in 6mins in ideal lab conditions.
Remember also that total burn times are only applicable if simmering style of heating is wanted – if the objective is boiling water to sterilise that’s a different application. Because esbit burns non linearly and tapers down fast in heat output in the last 50% of total burn time, so you need to tune the fuel load size according to your application eg different for front loaded heating in a short time if using to rolling boil, versus extended duration and lower heat output for longer if wanting to heat and then simmer.
The fast hot application for boiling water – I find it’s better to go with two 14g cubes at the start – it’s more effective for a rolling boil than a reload halfway due to the on linear burn and greater heat output of 2 in their first 50% of burn phases. You are left with residue of about 3/4 of a cube to dispose of when finished though. A snow leopard or fissure cone are best in these bush conditions and for boiling.
If it’s a coffee using already clean water, you don’t need a rolling boil, just heating the water, so half the fuel, and about 7-14g cubes are good enough even in the real.
Of course, if just heating then simmering if the goal, then less fuel is needed, it’s better to go cube by cube as needed, and The epicureans etc by Jon are a good way to go for those types of cooking.