The chemist must supply the right needle size and he should be able to get them within 24 hours. Don't accept the larger size as 4mm is now pretty well the standard and means less injecting pain. Go to another chemist if necessary and remember the chemist MUST dispense what is prescribed.
My chemist did this to me. I complained. They got me the right needles the next day but I'd still had to use some of the longer ones in the meantime. I bruised more with the longer ones.
I've been on 4mm needles for a long time now. I find 6mm needles cause bruising and can be a bit painful (I am a wuss mind).
Can still remember when 1st diagnosed, 46 years ago, the needles as long and as blunt as a darning needle. 4mm needles I can hardly see and have not had many painful jabs. Defo get the chemist to get the correct needles as on your prescription
I've used anything between 4 and 8, and it doesn't make much difference to me. Exept with Lantus, that stung less with longer needles. Might be different when you're skinny though.