The slow release versions of Metformin do tend to be quite big, due to the mechanism used to deliver the drug over a prolonged period. However, I have noticed that the size varies quite considerably between brands. The biggest I have encountered are Glucient SR, which seem vast; but this month they've sent me Metabet SR which are the smallest I've had, and are not much bigger than a 500mg Paracetamol caplet: a bit shorter in fact but also a bit thicker. You could ask your doctor to specify those to see if you can manage to swallow them.
I'm fortunate in that I can manage any size of tablet, but I know what its like for you as my wife has big problems swallowing even small ones.
She uses a technique she was taught: it sounds impossibly silly, but it works! : squeeze your ear lobe hard between finger and thumb as you swallow, and you'll find it helps a lot.