The drum is a 45 gallon the dimensions are 88cm x 59cm.
The exhaust for the cooking chamber is 9.3cm in diameter and 48cm in length (can add more or less length as only the external portion has been welded up).
The firebox (gas tank) is 41cm x 30cm.
The outlet chimney from the firebox is 12.5cm in diameter and hasn't been cut to size yet but will probably be around 18cm in length.
For the air inlet opening on the firebox I was going to make a rectangular opening (easier to cut) and I was thinking something like 10cm x 5cm or 15cm x 3cm.
Hope those are the dimensions you meant.