IMO a good general rule for large goanna enclosures is 4x total length x 2 times total length(this is not a strict minimum). You would also want it to be around 2m high to allow climbing, the easy thing with lace monitors is that they dont burrow like many other large species so dont require as much to stop them digging out. You can get away with keeping them in much smaller enclosures, especially if well decorated but they really should be given as much space as is practical. Generally an avairy style enclosure is best although it is possible to keep them in open top pits.
I would be interested in hearing about how people construct open top pits for large goannas so they are escape proof.
Lacies are the most common large monitor and lots of people have posted pics in previous threads although you will have to sift through all the stupid threads full of useless people saying search or google it...