# Calculating units in a cross currency short trade

If I have a forex account with a broker and a balance of 100 USD, and I'd like to short EUR/JPY, how many units can I short? How is this calculated? Which currency pair do I use to translate between my account balance in USD and the currencies involved in the EUR/JPY instrument? Which price (bid or ask) do I use in each part of the calculation?

(Please assume I'd like to use my entire USD balance and I'm not using leverage.)