Learn Italian: San Marco è il gran patrono di Venezia. - Saint Mark is the great patron of Venice.

When the title Santo comes directly before a masculine name starting with a regular consonant (like the M in Marco), it drops its final syllable and becomes San. If the name started with a vowel, it would be Sant' (like Sant'Antonio), and if it started with an s plus a consonant, it would stay Santo (like Santo Stefano).

Why do we use gran instead of grande in this sentence?

Just like Santo, the adjective grande drops its last syllable when it comes before a noun starting with most consonants. It becomes gran before both masculine and feminine singular nouns. Since patrono starts with a p, grande naturally shortens to gran.

Is it wrong to say il grande patrono instead of il gran patrono?

It is not grammatically wrong, but it sounds a bit less natural. Shortening grande to gran before consonants is extremely common in everyday Italian because it makes the sentence flow much faster and smoother.

Is patrono the standard word for a restaurant patron or a boss?