I have a 6.5 inch cock (when erected) so length should not be a major concern for achieving selfuck.
Anotherthing that may have significant importance is the "deepness" of the anus hole from the buttocks.
If you have it at the surface of your buttocks than it's easier to put your cock head inside it, and then the following length.
If your anus is deeper than the surface between your buttocks, than you will loose a few cms to put your head close to your anus and this might be critical to achieve selfuck.
I didn't succeed selfuck.
Here is to show you what I mean (the picture is taken after strong dilatation).
I tried anything else this evening : using a dildo to have prostate stimulation.
This dildo is thicker than my cock.
First cms were inserted with a little pain but later it seemed smooth and sweet.
That was strongly stimulating the prostate when moving in the right direction and even if I didn't have an orgasm, I saw a few drops of cum flowing from my cock head.
I'm sure that with long enough stimulation I will reach some interresting sensations...
Regards.
But seriously, it gets in the way sometimes.... even when I'm playing with toys. However, I know a lot of guys who are very envious of my foreskin.. and I have a lot... So I do nothing about it, other than play with it of course... 
