"Wilderland" is just an English translation of Sindarin "Rhovanion", which appears on the Lord of the Rings map written across Mirkwood and the northern Brown Lands. We can get a rough idea of the extents just by looking at the map and assuming that any major areas (named with the larger lettering) beyond Rhovanion are not a part of it.
For more specific details we need to look at other sources.
The index to the Silmarillion defines it as:
'Wilderland', the wide region east of the Misty Mountains.
That establishes it's western extent (the Misty Mountains) and it's also obvious from the LotR map that it ends in the east at Rhûn.
For north/south we can look to The History of Galadriel and Celeborn in Unfinished Tales:
Therefore after long journeys of enquiry in Rhovanion, from Gondor and the borders of Mordor to Thranduil in the north...
So that establishes a southern border at Gondor and Mordor, and that it extends north as far as the realm of Thranduil and the Mirkwood Elves.
The Wikipedia article on Rhovanion defines it as:
...a large region of northern Middle-earth. It extended to the east as far as the inland Sea of Rhûn; north to the Grey Mountains and Iron Hills, home of the Dwarves; west to the range of the Hithaeglir, or Misty Mountains; and south to the line marked by the Limlight river, Anduin, Emyn Muil, Dagorlad, and the Ered Lithui.
Although Wikipedia doesn't cite it's sources for this description, it does seem quite reasonable based on the maps and texts.