General rule-of-thumb is to not take advice from a shop worker, regardless of how knowledgeable they are or they seem. If you ask questions like "are they big?" and "are they compatible with (other species)?" the answer will almost always be the one that makes the sale seem as appealing as possible.
Rasboras are considered to be schooling fish so should be in larger groups, thus requiring a larger tank size (20 gallons would be a minimum imo). They can grow anywhere between 2 inches to 6 inches, max size isn't always the exact size each one will grow to, just a good number to go by to make sure you've got as much space for them as possible. Rasboras (like tetras) are generally peaceful and iirc can live in more diverse water parameters (particularly water hardness) than tetras and from my experience would be fine along with guppies.