As long as your arm
Definition 1 of 1
Pro Tip 1/3Exaggeration
Used to emphasize that something is very long, often more than expected or manageable. 
The shopping list for the party was as long as your arm.
Pro Tip 2/3Colloquial Usage
Widely used in casual conversation rather than formal writing or speeches. 
I have a to-do list as long as your arm this weekend.
Pro Tip 3/3Metaphorical
Not a literal measurement; it means 'very long' but doesn't specify actual length. 
Her wedding preparations included a to-do list as long as your arm.