Why didn't they leave more room for Beerus strength to grow?

[Galan007]

IIRC it was stated that Sidra was the fastest at creating shields, not that his shields were the strongest. I really doubt that blast would have killed any of them had it hit, maybe hurt them somewhat.

Perhaps. I was mainly going by Geene's statement: "Sidra... You saved us all."

Spoiler:

Which implies that if Sidra hadn't manifested that barrier at the last second, they all would have been screwed. /shrug