Here is a link to the futures contract specs. https://www.theice.com/products/197/EUA-Futures63
The contract is sized at 1000x the quoted price so currently is a +40k euro notional size for one contract. As with all futures you only need post the margin to hold the contract so that will be specific to your broker. Another way to think about the size of the contract and what it's daily movements would mean in your account. Each 1 euro movement will cause the price of 1 contract to vary by 1000 euros. The current daily movement in ATR is around 1.40 euros so the daily impact as of now would be around 1400 euros.