Well, we could make some security tanks or, if you prefer, just let these advance.  Your call.
		
		
	 
The Laser Security Tanks are advancing!
Oh heck, I'll just put in the conventional 5-8 HD (Medium); 9-12 HD (Large).
They look about a Challenge Rating 4 to me.
It'll be easy enough for a DM to make them BIGGER if they want to.
Updating the 
Guardian Droid Working Draft.
Hmm, think about bigger...
Mystaran Gargantuan Guardian Droid
(Aka Laser Security Tank)
Gargantuan Construct
 Hit Dice: 32d10+60 (236 hp)
 Initiative: +3
 Speed: 80 ft. (16 squares)
 Armor Class: 22 (-4 size, -1 Dex, +17 natural), touch 5, flat-footed 22
 Base Attack/Grapple: +24/+49
 Attack: Laser +20 ranged touch (12d6/19-20 25d6 plus DC 49 Fort save or instant death) or slam +33 melee (4d6+19)
 Full Attack: Laser +20 ranged touch (12d6/19-20 25d6 plus DC 49 Fort save or instant death) or slam +33 melee (4d6+19)
 Space/Reach: 20 ft./15 ft.
 Special Attacks: Laser, trample 3d8+19 [
Reflex DC 49]
 Special Qualities: Construct traits, damage reduction 5/adamantine, radarsight 600 ft. [
max 10 mile, detectable 200 miles], radiospeech 200 miles, robot traits, tracked chassis
 Saves: Fort +10, Ref +9, Will +10
 Abilities: Str 36, Dex 8, Con —, Int 10, Wis 11, Cha 1
 Skills: Listen +37, Spot +37
 Feats: Alertness (B), Cleave, Great Cleave, Improved Critical (laser), Improved Initiative, Improved Natural Weapon (laser), Improved Sunder, Overwhelming Critical (laser), Point Blank Shot, Power Attack, Precise Shot, Weapon Focus (laser)
 Environment: Any land or underground
 Organization: Solitary
 Challenge Rating: 18
 Treasure: None
 Alignment: Always neutral
 Advancement: 33-64 HD (Gargantuan); 65-96 HD (Colossal)
 Level Adjustment: —