There are Various Sizes

MT (Mini Tower)
DT (Desktop)
MFF (Micro Form Factor)
SFF (Small Form Factor)
USFF (Ultra Small Form Factor)

Beware of DELL OPTIPLEX (N-SERIES) which DO NOT HAVE AN OS.
The n Series is a Dell product line that does not ship with a pre-installed version of Microsoft Windows. Apparently prohibited from shipping computers without an operating system by an existing licensing agreement with Microsoft, Dell instead ships these systems with either the open-source FreeDOS operating system or the Ubuntu Linux distribution not preinstalled, but on install disks.
In addition to these Designators there are Some Series that only have 3 or 4 Sizes instead of 5

Regulatory Model and Type

Dell OptiPlex 7060 Tower, Model D18M,  Type D18M005
Dell OptiPlex 7060 Small Form Factor, Model D11S, Type D11S004
Dell OptiPlex 7060 Micro Form Factor, Model D10U, Type D10U003

 

Dell OptiPlex 9020 Mini-Tower, Model D13M, Type D13M001
Dell OptiPlex 9020 Small Form Factor, Model D07S, Type D07S001
Dell OptiPlex 9020 Ultra Small Form Factor Model D06U, Type D06U001
Dell OptiPlex 9020 Micro Form Factor Model D09U, Type D09U001

There was also a FLEX designation

11 Year Old Models from 2008
Regulatory Model (Mini-Tower): DCSM or DCSM1F
Regulatory Model (Desktop): DCNE or DCNE1F
Regulatory Model (Small Form Factor): DCCY or DCCY1F
Regulatory Model (Ultra Small Form Factor): DCTR or DCTR1F

http://www.dell.com/downloads/global/corporate/environ/comply/optix_745flx_dcsm.pdf
http://www.dell.com/downloads/global/corporate/environ/comply/optix_745_dcsm.pdf

Models Like GX1 or GX110 (20 years Old)
MMP = mini tower (MT) chassis (Trapper) 200 watts power
DCM = midsize desktop (M) chassis (Hunnicutt) 200 watts power
DCS = low profile (L) chassis (Hawkeye) 110 watts power
DCP = small form-factor (SFF) chassis (Gator) 085 watts power

 

 

 

Newer Has TOWER SMALL and MICRO

 

Micro Form Factor

 

 

What do they Look Like

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

960 970 980 3010 7010 9010 3020 9020 Etc

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DELL FREEDOS