As a general rule, the dose of Doxy for a dog is 2-5mg per pound given twice a day.
So if your dog weighs 50 pounds, they'd get anywhere from 100-250mgs twice daily.
There's more information about Doxy here: http://www.1800petmeds.com/Doxycycline-prod10340.html
I hope this helps.
If given twice a day, that's only 650mg per dose, or 4mg/lb....which isn't high at all.
Since the doxy is often prescribed as twice daily dosing, that would be the normal dose for a dog his size even if it's only given once a day.
I wouldn't worry about the pepcid...but giving it with food can help with the digestive upset that is really common with any kind of antibiotic.
The way the dose is distributed is dependant on the vet...some prescribe twice a day dosing, and some prescribe once a day. Protocols can vary from clinic to clinic even in the same town, and sometimes can vary between vets in the same clinic!
If you feel like she's getting digestive upset from the Doxycycline, then you can break it into two doses, but if she's not getting sick from the way the vet prescribed the medications, then I'd just follow their directions.