I'm building a barrel type oven using half fire bricks, split in half, the fire brick is built up. The external dimensions from fire brick only are

External 30L x 25.5W 16.5 h

Internal (cook area): 27.5 L x 22.5 w x13 h

Its a smaller oven so in comparison to a full brick oven, how much insulation is needed/recommended? Some Questions:

1. Is the fire brick alone enough to handle the temperatures needed, that is, would fire blanket or added insulation be redundant or overkill given the size of the oven?

2. I bought 2 in fire blanket, but not sure if its worth adding the extra thickness because I prefer a lighter/smaller oven. If it definitely worth using, then I don't mind. 

3. oven internal spec is 27.5 L x 22.5 w x13 h. So the opening is 22.5 x 13h barrel style. What is a preferred opening for variety of dishes/cooking? I can close the opening/face more, but I don't want to go too small.

Thanks for feedback if you're able.