From the Fallout Wikia site notes and each Mini Nuke is the size of a football... I also remember reading that the exploding vehicles of the game due about the same damage as a Mini Nuke.
The wikia states the damage done is 10/80 for FO3 and 400+ for New Vegas
Fallout 3
Legend
Damage DPS AoE Special APS Critcial Critical Damage AP DAP Weapon Spread Magazine Durability Weight Value {PDC Value/Weight Ratio
Fat Man 10 2542 1600 +10 Rads/12s 1.58 x0 0 65 24.8 2.0 1 167 30 1000 33.3
Experimental MIRV 80
(10x8) 20336.2 12800 +80 Rads/12s 1.58 x0 0 65 198.2 13.0 8(1) 63 30 2498 83.3
New Vegas
Damage DPS AoE ApS Critical Critical Damage AP DAP Weapon Spread Magazine Durability Weight Value {PDC} Value/Weight Ratio Skill Strength
Fat Man 400 1578.9 600 1.58 x0 0 65 15.4 0.5 1 495 30 6000 200 100 8
Fat Man {+Mods} 400 1578.9 600 1.58 x0 0 65 15.4 0.5 1 495 15 8500 566.7 100 8
Fat Man {GRA} 400 1578.9 600 1.58 x0 0 65 15.4 0.5 1 495 30 8000 266.7 100 8
Fat Man {GRA} {+Mods} 400 1578.9 600 1.58 x0 0 65 15.4 0.5 1 495 15 10500 700 100 8
Esther
475 2036.8 600 1.89 x0 0 55 19.5 0.5 1 620 40 18000 450 100 8