How do I know if my meter tails are 25mm?

This is a job that should only be undertaken by a qualified Part P registered electrician, they will know what size tails and earth to fit.

IF you are getting your main fuse upgraded, they will need to ensure the tails will cope. So they should be updated at the same time. There are more things to take into consideration while this is being done so I most strongly recommend you set on a qualified electrician to get all of this sorted in one go. At least he will sign these upgrades off and certify the work that you need to doing that prompts you to increase the main fuse rating. That is the question that would worry me. Because for some reason you feel the need to increase the fuse rating which would get me to ask WHY. Then with a reasonable answer I have no doubt you are looking at a new distribution board as most 80/60 amp boards are very limited and have no rcd protection at all. I would not get ANYTHING upgraded until the job required has been inspected and explained by a qualified electrician.

Yes there is. The old saying ask an electrician to have a look. Having said that, it appears that you are fiddling around with suppliers equipment, even us who are registered electricians are not allowed to do it, if caught you could face prosecution by the supplier. How do you know that the supply cable is big enough to take a 100amp load? I, even as an experienced spark of 40 odd years would not even dream about doing this. Even if it is not the public suppliers equipment, if you can’t tell if the meter tails are 25mm or not, I’m sorry but you should be nowhere near this.

Really get an electrician