I think it depends on the chin and how big the bars are both ways. I had a boy in the FN from the first day I got him. He was a slim 560 grams when I first got him at about 9 months (he was eating bad food, etc.) and his largest weight was about 720 grams. He was way too large to fit out the FN cage bars.
My current girls are approximately 250 grams and 275 grams. Pixie, the larger, is of a stockier build and I am not sure she could escape but Trixie who is slimmer might be able to. I wouldn't risk it however with either of them. I plan to take the hardware cloth off after a few months when they are too large to escape.
For less than 5 dollars I was able to get enough hardware cloth to wrap the whole first level. I got it at Lowes and it's 1/2inch by 1/2inch squares. I think 1 inch is much different than 1 1/2 inches when it comes to a chinchilla. When I used the hardware cloth I had plenty left over to do the bottom level too if I had needed to. If you were local to me I'd give you the rest of the roll! Just be careful it is very sharp when you cut it. We used garden gloves to work with it.