EDIT: I read the original article in the opposite way and described the largest PNG file with single pixel, oops. It's still a good tidbit so here is the original comment.

There is a much easier way. You can put an arbitrary number of empty uncompressed blocks, which are normally used for the sync flush (Z_SYNC_FLUSH in zlib). Such blocks do not affect any other block or zlib checksum (but do affect PNG checksum). For example a DEFLATE stream `7801 (00 0000 ffff)* 01 0100 feff xx yyyy yyyy` is always valid and decompresses into a single byte `xx` where `yyyy` is a big-endian representation of `xx + 1`. Also PNG IDAT chunks have to be implicitly concatenated, so you can technically have an arbitrary large PNG file of single pixel if you really want.