My SO says my cock is not long enough for us to do doggie.

How do I tell her, it's not my cock.....I've had several great doggie sessions providers ? She need to move her ass a little.

Unless you can go under an alias you can just merely say "Well Ive thought about it and lets try it by you....blah blah

OTOH, you might have a SO who has gained some weight on the hips/arse/thighs over the winter. You're not shorter, the trip to heaven just got longer.
You might want to alter positions with a leg up or go through some variations on any of the "sex positions 101" type sites.

OTOH, you yourself might have gained some weight which can add a bit of padding to your pubic area and thus, you really are shorter.

or worse, both of the above!

Either way, I recommend you two working out a bit more - now that the weather's better.
Active, yet still romantic walks can help your lives and relationships, in more way than doggie style.

My two cents

Kosh

Tell her to lose some weight ! With less junk in her trunk , your junk will fit.

Correct ! And if that doesn't work , you'll just have to tell her the truth and that's that her Ass is too large and she needs to lose weight.