Some Italian adjectives have the same form for masculine and feminine singular. Forte is one of them.
So you get:
In the plural, it becomes forti for both masculine and feminine:
So forte is correct with puzza, even though puzza is feminine.
It is pronounced roughly POOT-tsa.
A few useful points:
So puzza is approximately POOT-tsa, with stress on the first syllable.
This is the normal word order in Italian: subject + verb + adjective/complement.
So:
This matches English fairly closely: The stink is strong.
Italian can sometimes change word order for emphasis, but La puzza è forte is the standard, neutral order.