I've had the same sim card about 8yrs
I'm sure your right about about making a SIM card smaller.buy the standard card . And then make it 1 or 2 sizes smaller.
If your old mobile is a sim free phone.then yes any pay as you go sim card will work.if say your old mobile was on a certain network then you will need one of their sim cards .as sim cards come in all shapes and sizes🤔
£4 pound in poundland buys me pack of ham slices£1 pack of cheese slices £1 pack of cakes£1-50 should last a week sorted.plus 50p for one sip of larger.