I have a 7.5lb female mini daschund and I want to breed her to a 18lb mini daschund. Can this be done? Does the size difference matter? Thank you.

Not a zoology question really - except in the broader sense. There is a Pets and Animals section in the Home and Garden area where the pet experts gather.

Or an even better option, check out the hundreds of other 'i want to breed my daschund' threads by using the search option at the top of your page.

I've said it time and time again, if you don't know about breeding dogs then you shouldn't be breeding.
The question you have asked is very basic to someone who has knowledge of breeding and therefor I personally will not answer it.