SIM Card Size for New Product?
You seem to have left a gap in your list and sim card names can be a bit confusing. So for clarification Sim cards come in the following sizes


full-sized sim (85.60mm x 53.98mm): credit card size. Used on some really early phones afaict.
mini-sim (25mm x 15mm): this was the standard for many years and often gets referred to as a "regular sim" or similar, presumably because hardly anyone remembers devices that took the credit card size sims. 
micro-sim (15mm x 12mm): used for a while smartphones and tablets
nano-sim (12.3mm x 8.8mm): seen in more recent posh smartphones and tablets.


The last sim I obtained in the UK came as a breakaway card that supported all sizes from the ancient credit card size down to nano sim

Given that the networks still seem to be supporting the credit card sized sim that no new phone has used for many years I wouldn't worry about availability of sims and would just go with whatever you think your users will find easiest to work with.