The file sizes are all true file sizes (Data only), and not "Size On Disk" sizes.

Game: Doom SE - Special Edition a.k.a. (Ultimate Doom 1995)
File: DOOM.WAD
Size: 11,8 MB (12 408 292 bytes)

Game: Doom II - Hell on Earth (1994)
File: DOOM2.WAD
Size: 13,9 MB (14 604 584 bytes)

Game: Doom II - Plutonia (Final Doom)
File: PLUTONIA.WAD
Size: 17,3 MB (18 240 172 bytes)

Game: Doom II - TNT (Final Doom)
File: TNT.WAD
Size: 17,7 MB (18 654 796 bytes)

Game: Heretic - Shadow of the Serpent Riders
VERSION: 1.3
File: HERETIC.WAD
Size: 13,5 MB (14 189 976 bytes)

Game: Hexen - Beyond Heretic
VERSION: 1.1
File: HEXEN.WAD
Size: 19,1 MB (20 083 672 bytes)

Game: Hexen - Deathkings of the Dark Citadel
VERSION: 1.1
File: HEXDD.WAD
Size: 4,23 MB (4 440 584 bytes)