Knight,
Wherever "sex problems start" it doesn't sound as if you have anything to worry about; so far as I know many first times are nothing like as good as what you have described.
The way you framed the question highlights a distinction. A problem with insertion is one thing, and it must relate to girth issues, or maybe just the size and shape of your glans if you are talking about just gaining entry at all. A problem with complete insertion to full depth is another matter, and brings in the length issue.
I suppose quite a few gents here with very long penises will be able to give a good guide as to where length problems start, simply from their knowledge of how much they can use, especially if they have had a lot of partners. My own limited experience corroborates what Steve26 said; most men up to 8 inches will be able to use their whole length with many women, but as you go above that length you can increasingly expect to have to hold back. I have a pretty precise idea how much I can use from the 'doughnuts' I wear from time to time when I think I am not going to be able to restain myself. In my case I usually stop at 8" or maybe 8.5" even though I have on occasion gone beyond.
That puts you at pretty well the ideal size.
Initial penetration and girth are another matter. It will depend a lot on the woman and her own experience, and her state of readiness. But also on you, not only your girth but maybe even your shape. Again I would guess (and it is only a guess), that a man over six inches in girth will have to be much slower and much gentler at least to start with. Well experienced women, espcially those who have enjoyed a number of well endowed partners, would be the best people to answer this question. The trouble is, few men (let alone their partners), have a really accurate idea of what their girth actually is.
Yet there are many big men on here who say that they can achieve very deep penetration. It may be they have the good fortune to have found a very receptive partner; there could also be other factors. Some erections have more give in them, maybe they are little more spongy or flexible, or they are curved differently. In the end you can't know until you (gently!
try.
Sorry for the long post.