We've discussed the Emperor Diocletian before (see Diocletian Has A Very Good Day) and it was during his reign (284-305) that the largest public baths in Rome were built. The baths covered an area about 1200x1000 feet or about 30 acres, accomodate 3,000 people at one time and included cold, warm and hot baths along with swimming pools, libraries and meeting halls. Below is a reconstruction of the baths during the 4th century (from maquettes-historiques) at a time when Rome still had nearly a million inhabitants:
By the 6th century the baths were out of use as the City of Rome declined with the end of the empire. Along with much of the rest of the city, the area around the baths was abandoned except for scattered dwellings, vineyards and farms. This is a painting of the baths from around 1700 by Pieter van Bloemen (from wikipedia).
From later in the 18th century we have a painting of the cold baths (the Frigidarium) by Giovanni Piranesi (from Indianapolis Museum of Art).