When to upgrade for a bigger terarrium?

My ball python has been growing quite quick! He's going on 7 months old here soon.The last time I weighed him, he weighed 154 grams. I've had him sine August 28th and he's been kept in a 20 gal terrarium (12x18x24).
When should I get him a bigger terrarium? How do I measure him if he keeps moving? When I upgrade, should I shoot for a 30 gal or a 40 gal terrarium?

Anything helps!:D

Pictures of the snake in the enclosure might help. What is his current weight? You can bump him up to a larger enclosure just make sure there are a few extra hides some foliage and some crowding so he isn't overwhelmed.