depends on your car matey.
If it's an A3, your MAF is 70mm, if it's an S3, your maf is 80mm.
If you intend to stick with a ko3 or ko3S on an A3, then stick with the 70mm filter. If you intend to upgrade beyond a ko3S in the future and hence a larger 80mm MAF, you could get the 80mm neck filter and use a reducer for now, thus future proofing the filter for further upgrades.
if it's an S3, then the 80mm filter goes straight on.
When I had a ko3S I ran an 80mm filter with a coupler and a reducer down to 70mm for my MAF, this pushed the filter further out into the inner wing as well. was a good setup.