What size backpack is right for me?

HERO 2.0 45L Backpack Hero 2.0 25L HERO Backpack
48cm x 34cm x 29cm 42cm x 30cm x 14cm
Pair of Trainers Pair of Trainers
17'' Laptop 15'' Laptop
Large Towel Towel
Change of clothing Change of clothing
Drinks Bottle Drinks Bottle
Wash Bag Wash bag
Wrist Wraps Wrist Wraps
Knee Sleeves Knee Sleeves
Skipping Rope Skipping Rope
Snacks Snacks
Headphones Headphones
Pair of Weightlifting Shoes 2 x 750ml drink bottles
Lifting Belt  
Meal Preparation Box  
Protein Shaker  
2 x 750ml drink bottles  

45L Size Reference

25L Size Reference