Who made the gigantic beer sign at seals stadium in San Francisco and where is it now?

The gigantic beer sign you're referring to is actually a Budweiser sign that stood above Candlestick Park, not Seals Stadium.

While the exact creator of the sign is not widely documented, it was likely built by a company specializing in large-scale signage and advertising.

Unfortunately, the Budweiser sign was demolished when Candlestick Park was torn down in 2014.