Hey guys,
I was researching about texel density for a personal project in UE4 and I bumped into a couple of interesting threads (including this one) that can help about the topic.
In particular in this one here Standardizing Texture Density TeriyakiStyle proposed to standardize the texture density measures for UT environment art.
Based on those information and on CharlestonS reply I wrote down for myself the following notes (cause I am little dumb with math and proportions) 
Note: bear in mind that the measure in cm have been rounded and are not exact (because have been converted from the feet measure): they simply helped me to have a rough idea about the measure (because I am not accustom to feet units).
[UDK/UE3]
In UDK/UE3 the scale was the double as it is in UE4.
1 unit = 1.95 cm
1 foot = ~30 cm = 16 units
Texel Density: 8 pixels x unit
• 8 units = 0.5 foot = 15 cm = 64x64
• 16 units = 1 foot = 30 cm = 128x128
• 32 units = 2 feet = 61 cm = 256x256
• 64 units = 4 feet = 121 cm = 512x512
• 128 units = 8 feet = 243 cm = 1024x1024
• 256 units = 16 feet = 487 cm = 2048x2048
• 512 units = 32 feet = 975 cm = 4096x4096
[UE4]
In UE4 the scale is half as it was in UDK/UE3.
1 unit = 1 cm
1 foot = ~30 cm = 30 units
Texel Density: 8 pixels x unit
• 8 units = 0.25 foot = 7,6 cm = 64x64
• 16 units = 0.5 foot = 15 cm = 128x128
• 32 units = 1 foot = 30 cm = 256x256
• 64 units = 2 feet = 61 cm = 512x512
• 128 units = 4 feet = 121 cm = 1024x1024
• 256 units = 8 feet = 243 cm = 2048x2048
• 512 units = 16 feet = 487 cm = 4096x4096
This should answer the initial question I hope. 