Hi auntaanne, I am sorry to hear that you are going to have to modify all your sock patterns.
It's not that hard, if you've already made a sock or two, to add some stitches and fudge some numbers. But since you haven't knit a sock before, it can be quite confusing.
Do you need more length, more width, or both?
For a wider foot, you get a more comfortable sock if you cast on a few extra stitches rather than go up a needle size. (Going up a needle size means you will feel every purl bump under your sole.) If the extra stitches don't fit into the stitch pattern repeat, you can work them in plain stockinette.
For a wider ankle, you can go up a needle size (looser fabric on the leg won't affect comfort and wear the way a larger gauge on the foot would). In fact you can go up a needle size every few inches to shape your calf.
For more foot length, add rows between the toe and the gusset.
Socks will stay up if they have about 10% negative ease in the ankle and leg. (Another cause of socks creeping down is a too short foot, or a toe box that's too narrow.)
Also, if you can start with a toe-up pattern, you can fiddle the stitch count for a perfect fit as you go ... for example you can stop increasing the toe when it fits over all but your smallest toe, which gives you just the right amount of negative ease.
And use one long circular needle if you can, because you can try the sock on your foot at any time without having to move the stitches to a length of string.
Knitting socks is fun and rewarding and I hope you have a good experience with your first pair.