My dog is very big.

Моя собака самый большая.

"Stressed Singular Nominative.

A feminine form of a stressed adjective is the same as that of an unstressed adjective - it will end in -ая-.

NB. The Russian word for big is большой. Because the ending is stressed the letter ш can be followed by the letter -o- this applies both to the masculine singular and neuter singular." - Essential Russian Grammar, Daphne West.