Rubblehulk's power and toughness are each equal to the number of lands you control.
Bloodrush — {1}{R}{G}, Discard Rubblehulk: Target attacking creature gets +X/+X until end of turn, where X is the number of lands you control.
If you activate Rubblehulk’s bloodrush ability, the value of X is the number of lands you control when that ability resolves.
The ability that defines Rubblehulk’s power and toughness works in all zones, not just the battlefield.