I only get the first three numbers followed by periods ("123..") for my Watch paired with my iPhone 5.
How do I find the full four or five digit device account number?
What full account number are you referring to? And where are you looking?
I'm looking under Passbook & Apple Pay on my linked iPhone 5 My Watch" app.
According to the Apple Watch User Guide, page 58 https://manuals.info.apple.com/MANUALS/1000/MA1708/en_US/apple_watch_user_guide. pdf only the last 4-5 digits of the credit or debit card number will appear on the front of the payment card.
I only receive three with the final numbers depicted similar to:
123..
the the last two numbers are omitted as periods.
It very well may be the fact the watch face is rather small. Either way, it is not intended to display the entire number.
if I don't have the whole number, how can I use it when prompted by the merchant, I haven't used Apple Pay before I got my watch because my iPhone 5 didn't support it.
The merchant does not need the entire number. The whole purpose of Apple Pay is to protect the card number. Apple Pay uses a device specific number when used that masks the actual account number to prevent the possibility of identity theft. The merchant should not be asking your for the number anyway if they are using NFC for payments. You might want to take a look at this FAQ describing how Apple Pay works. There is a section on how it handles account numbers. Apple Pay FAQ - Apple Support
Try going to Settings>Brightness & Screen Size and making the text smaller.
Device account number for watches paired with iPhone 5
