The margin borrow limits for each currency varies based on your current verification tier. Note that each limit is independent from the other, so a Tier 3 user could, assuming there are funds available to lend, borrow up to 25,000 EUR and 25,000 USD at the same time. Please refer to the following table:
|Tier 1||Tier 2||Tier 3||Tier 4|
If you try to open a position with margin that exceeds your borrow limits you will see a "margin allowance exceeded" error.
Your account is Tier 3 and want to open a long position in bitcoin against fiat currencies. The maximum size of your position, assuming you had enough collateral to open it, would be 25,000 USD worth of bitcoin plus 25,000 euro worth of bitcoin. The number of bitcoins that it would be possible to buy in these positions would depend on the price of the current price of bitcoin. The limit would not be 50 bitcoin (the tier 3 limit for borrowing bitcoin).
If you wanted to open a short position in bitcoin against USD, EUR, or ETH, the limit on the size of the position would be 50 bitcoin because when you're shorting bitcoin, you're borrowing bitcoin from Kraken and selling it.