Generally you don't set a filesize ahead of time, it is determined by what resolution your photo is and what JPG quality you choose. (JPG quality is the amount of compression - higher compression means a smaller file but worse image quality.) You can also save it without exif or iptc data to make it slightly smaller.
That being said, the latest version of Irfanview does have the ability to let you choose a file size. Open an image, select Save As, and one option is to set the file size.