315 ILCS 20/24
Neighborhood Redevelopment Corporations subject to the supervision of the Redevelopment Commission of cities, villages or incorporated towns having a population of five hundred thousand or more shall not possess at any time a Development Area less than two city blocks nor more than 160 acres, and Neighborhood Redevelopment Corporations subject to the supervision of the Redevelopment Commission of cities, villages or incorporated towns having a population of less than 500,000 shall not possess a Development Area less than one city block nor more than 40 acres, unless the Redevelopment Commission, in the interests of the orderly Redevelopment of a Slum and Blight or Conservation Area, shall, after a hearing upon its own motion or upon application of the Neighborhood Redevelopment Corporation, authorize a reduction or increase of the Development Area. It shall be in the interest of the orderly Redevelopment of a Slum and Blight or Conservation Area to increase or diminish the Development Area herein limited whenever the Development Areas of two or more Neighborhood Redevelopment Corporations operating or to operate in the same locality shall not encompass an intervening area of less than two city blocks, and the Redevelopment Commission in such instances shall by rule, pursuant to paragraph 2 of Section 25 of this Act, prescribe the conditions under which contiguity, and the extent thereof, shall be mandatory of the two or more Development Areas. The words "city block" as used in this Act shall mean a parcel or parcels of land bounded by and without intermediation of, streets, public waterways, railroad rights of way or other similar public spaces (not including alleys), or any combination of them.
315 ILCS 20/24