BBC: Ice Lake Found on Red Planet
The highly visible ice is sitting in a crater which is 35 km (23 miles) wide, with a maximum depth of about two km (1.2 miles).
It looks like the ice lake is roughly one third the width of the crater. Given the estimate of 35 km (23 miles) for the crater diameter the ice lake is probably easily 7 miles (11.2 km) in diameter itself which is a pretty decent sized lake. I tried finding a Spokane area lake of comparable size using google maps and most of them seemed smaller. I think the northern most section of Lake Coeur d’Alene in Idaho might be comparable though. It’s definitely a sizable deposit of ice though and could probably sustain some kind of base on Mars for a long, long time if one was located there.
Update: Even bigger pictures at the ESA(European Space Agency) site.