The biggest lake of Finland's 188 000 lakes is definitely Saimaa. Its area is almost 4 400 km^2 and it covers a big part of Eastern Finland.
It's not a one big lake though but more like several open lakes connected by narrows. However longest open lake area, Pyhäselkä, is almost 40 km long.
With highest depth of 82 m Saimaa is fourth deepest lake in Finland but coast line is found almost 14 850 km.

Outlet of Saimaa is Vuoksi, biggest river of Finland by the volume of water which is as big as 580 m^3/s. Vuoksi runs first 14 km in Finland and then 126 km through Russia and ends to lake Ladoga.

Saimaa is fourth biggest lake of whole Europe...