The standard units measure approximately 14.5" x 14.5"n x 23". 
The junior units measure approximately 11" x 11" x 16.5"
The Austin "it" unit measure approximately 11" x 11" x 8.375"

You can also check the label on your unit.
This may be on the back or the underside of your unit.

Standard units have codes that begin with HM4... for example HM400 or HM410, etc.
Junior units have codes that begin with HM2... for example, HM200 or HM210, etc.

The Austin Air Family Lineup - Standard Junior and it Air Purifiers
Austin-Air-junior-Air-Purifier-Speed-Switch Austin-Air-standard-Air-Purifier-Speed-Switch
If you have any questions about replacement filters and specifications, please contact the manufacturer directly.