This is what I used when buying bolts for my 7mm that didn't come with extended bolts. Pulled from Bimmerworld's extended bolt product page. Personally, I would err on the side of longer than shorter i.e. using extended bolts meant for a 15mm spacer on your 13mm spacer.
https://www.bimmerworld.com/Wheels-T...Bolts-M14.html
