There is no value known without knowing and resolving the specific application.
Each application has its own value - which normally (in my experience) is not even of academic interest.
There is certainly no general value ever tossed around in design circles. Why do you ask and what would you want to do with it - if you were given one?
I very much agree with Montemayor. Depending on your applications, L/D (or D/L- as you mentioned). I will give some rough guidelines: here I will interms of L/D (so if you are using D/L, invet the values):
For G-S reaction, if your catalyst crush strength is not a problem, then L/D = 5-6. However, I have designed L/D < 1 also.
In that case, make sure Pressure Drop across the bed is at least 5psi (typical High/Low Shift syngas application).
But some cases, pressure Drop might be high to meet the required L/D=5. In that case, you have to make sure proper flow distribution.
For G-L reactors, maintaining L/D is important, just to make sure channeling is not taking place.
For Fluidized bed, it depends what you want to acheive- for example, if you want to maintain particular residence time, your reactor will be long narrow, and some special distributor may be needed.
If you identify your application, then we can give some pointers.
For a reasonable rundown on tubular, fixed-bed and fluidized-bed reactors' design and optimization, I advise reading the relevant chapters in Ullmann's Encyclopedia of Industrial Chemistry, volume B4: Principles of Chemical Reaction Engineering and Plant Design (VCH)
