According to the gsm areana site (which is usually pretty accurate for this sort of stuff), the full spec details for each of these models are as follows:
OnePlus 6:
External Dimensions: 7.5 mm thickness (no data on length and height as of yet, until officially announced, but the screen size may give you some idea)
Screen Size: 6.28 inches, 98.4 cm2
Resolution: 1080 x 2280 pixels, 19:9 ratio (~402 ppi density)
https://www.gsmarena.com/oneplus_6-9109.php
OnePlus 5T:
Dimensions: 7.5 mm thickness
Screen Size: 6.01 inches, 93.7 cm2 (~80.0% screen-to-body ratio)
Resolution: 1080 x 2160 pixels, 18:9 ratio (~401 ppi density)
https://www.gsmarena.com/oneplus_5t-8912.php
As you can see the thickness of both phones appears to be the same, however the screen size is not. (the screen on the OnePlus 6 looks like it will be bigger than the OnePlus 5T). Though bear in mind if they're doing the whole full screen with notch thing, that doesn't necessarily mean that the external dimensions will change. So for that, we'll just have to wait and see.
If you could have the dimensions any way you wanted them though, what would you ideally be hoping they would be?