LED size nomenclature
From this page:  


  The T-1-3/4 nomenclature indicates the shape and size of that particular device. The T or various other descriptions (S, G, PR, TL, etc.) indicates the envelope shape of the device [...]. The 1-3/4 indicates the envelope size in eighths (0.125) of an inch. An example is a T-1-3/4 lamp has a 0.219 diameter (1.75 X 0.125 = 0.219).  


So T-1 is 1/8 inch, or (about) 3mm, T-1 3/4 is 1.75 \$\times\$ 1/8 inch, or (about) 5mm. A 10mm LED is T-3 1/4.