Does anyone know much on how to tell the species of garden slugs and snails?!

I have three slugs and a snail, I think the snail is a brown garden snails but not sure, and one of the slugs is possibly a banana slug but im not sure about the other two (they are the same)

They have black bodies with light brown lines down either side of the body!

Does anyone know how big any of these will get?

Sorry about all the questions but the internet isnt all that helpful :\

xx