Foam Rollers come in all shapes and sizes and are a great recovery tool. Different rollers provide different levels of hardness. A harder roller while provide a more intense massage whereas a softer roller will provide a more gentle and comfortable massage. 

The ideal length of rollers depends on its use. A longer roller can be used for almost any exercise whereas a smaller roller may restrict you to one leg at a time. A shorter roller however is easier to store and transport. 

The dimples of some rollers can provide a more hands on feel and have an effect similar to a spiky trigger point ball, penetrating the muscle tissue deeper and providing greater relief.
Half round rollers are ideal for static stretches and can be combined as a balance tool.