When the female is outside of heat or spayed, the vagina stays quite small. It enlarges during heat. Some larger dogs are able to have sex even out of heat, but penetration is more difficult. In all cases you need lots of lube and even larger dogs may need slow start so that the vagina stretches open over time.
I do not like this principle, that you have to work on stretching the opening in order to have sex, it seems kind of wrong to me, so I would go for a large breed if I ever wanted a female dog.