It's a bit more nuanced than just "large" as it implies a size that is noticeable but not necessarily enormous.
Here are some examples:
* "C'est une assez grande maison." (This is a fairly large house.)
* "Elle a une assez grande collection de timbres." (She has a pretty big stamp collection.)
* "Il y a un assez grand parking." (There is a fairly large parking lot.)