What do the sizes (Petite, Miniature, Medium, Standard) mean ?
My estimates on the adult size of a litter of puppies is just that, an estimate.  I do have years of experience, and many times previous litters from the same parents, to go on.  However, the range can be quite large.  Consider the two male puppies from the same (medium) litter who grew up to be 35 pounds and 65 pounds.  That is not common, but it can happen.  Below is the general size ranges for Goldendoodles.  Dog height is measured at the withers (on the dog’s back, above the shoulders)

Petite:  shorter than 14,”  usually under 25 pounds.

 Miniature (aka "mini"): between 14” and 17,"  between 25 and 35 pounds.

Medium: between 17” and 21," between 35 and 50 pounds.

               Standard: taller than 21,”  usually over 50 pounds.   

Goldendoodle Information

Goldendoodle Association of North America

What are the different types (generations) of Goldendoodles ?
According to the Goldendoodle Association of North America, a dog containing Golden and Poodle, in any combination, is a Goldendoodle.  Below are the generally agreed upon terms for the various mixes.    

               F1 Goldendoodle= Golden Retriever x Poodle

               F1B Goldendoodle= F1 Goldendoodle x Poodle

               F2 Goldendoodle= F1 Goldendoodle x F1 Goldendoodle

               Multigen Goldendoodle= Two Goldendoodle parents

               *One parent has to be an F1B or multigenerational 

               *A Poodle bred to a multigenerational Goldendoodle also constitutes a multigen.

There is a much more in-depth look at these types, if you are interested, on Goldendoodles.com