altering pattern from a size 9mm down to a 6mm

I am a good knitter but cant work out how to alter a pattern to suit the smaller gauge needle
I have jsut knitted a tunic dress fully and have discovered that it is too small.

the pattern said to use a 9mm needle but I thought this would be too big a hole so I used a 6mm
and have found that it is too small
pattern said to cast on 70 stiches using a 9mm I did the same stiches using a 6mm needle.

My question is how many stiches should I cast on

Welcome to the forum!
Patterns recommend a needle size but you need to determine which size needle will give you the pattern gauge (stitches per inch). The best way to figure that out is to knit a swatch. Cast on more sts than the given pattern gauge and work about 4 inches. Then measure across the center 4 inches to see how many sts per inch you’re getting with your tension, yarn and needles.