The simple answer there is no person too big, it is only your not prepared plenty of lubricant both around the anus and into it aswell, then loosen up the anal hole with his finger or yours. Then as your anal hole expands a little do not squeeze you bottom or you will tighten up it again.
Get your good fellow to put plenty of lubrication on his dick too.
Off you go and enjoy, that's what I do and have no problems.
Think about this one just imagine the size of some of your exits from your anus, and this little hole has to expand for this purpose, enjoy that's what I do.
Randy69