Seconding what everyone else has said so far - length is not a detriment to these devlogs. The more content, the merrier.
I would say though, if you're looking to optimize these devlog updates for maximum reader enjoyment, the key dimension to prioritize is neither length nor frequency, but rather regularity.
If people can expect a monthly / bi-monthly / quarterly update, the anticipation will go a long way to keeping people around and engaged.