Which needles?

I’ve been knitting a baby blanket in Sirdar Funky Fur. The pattern uses 4mm (UK8 - USA6) and 3 1/4mm (UK10) needles. The tension is 22sts and 32rows to 4" on 4mm needles. Overall blanket size is 18 x 26". My tension doesnt seem to be right and blanket is only about 15" wide. So following the pattern it says to use thicker needles if more stitches needed. I dont know how to work out which size needles to use. The pattern casts on 120sts on 3 1/4mm then after 12 rows decreases to 102sts and changes to 4mm needles.

You’re knitting about 26 stitches per 4", so I’d try 4.5mm needles (that should be 1 sizes bigger) and see how those work. (And go 1 size bigger on your 3.25mm needles up to 3.5mm.) You may need to go up 2 sizes to 5mm needles (and use your 4mm as your smaller pair), but I think the 4.5mm should make your blanket about the right size. Sometimes it takes some experimenting to get the right guage.

These are all commonly used sizes, so you’ll use them again in the future if they don’t work for this project.

Yeah go up about 2 sized to an 5mm. Keep the same number of sts, but the larger needle will make it a larger size.