UICollectionViewCell dynamic sizing for 4.4 and 5.5 inch

Question

I have UICollectionView with cells in the storyboard. The size of each cell is set to 145x145.

They look good on the iPhone 4 - 5s, but the size doesn't increase proportionally on the iPhone 6 and 6+. Rather than manually setting a different cell size for each device, how can I do it dynamically?

See examples here: iPhone 5S: http://prntscr.com/55vy7q, iPhone 6: http://prntscr.com/55vyy2