General dimensions and recommendations for a USB dongle
There is no standardization.

If the device is particularly large in dimensions or heavy in weights, then it would probably designed in such a way that having a USB extension cable is compulsory and shipped along with it.

Such as USB HDD External


It would be difficult and troubling for HDD having USB plug popping out at the top or at the side so that the user need to plug it directly onto the USB socket. It will probably interferes itself with the surrounding, falls because of the weight, or even damages the socket or the plug itself.



Take a look at USB Specification datasheet for each connector dimensions. 

Such as USB 2.0 or USB 3.0.

For example in USB 2.0 specification Figure 6-3. USB High-/full-speed Hardwired Cable Assembly at page 89. It is stated that USB type A receptacle length is \$15.7mm\$ and the width \$7.5mm\$.

Although there is a warning stated that 


  Dimensions are TYPICAL and are for general reference purposes only


So you could technically designed your device with typical maximum length and width of \$15.7mm\$ and \$7.5mm\$ respectively and then guesstimating your device height to suit your needs.


For example again, take a look at Asus USB-AC53 Nano specifications




  Dimensions : 20 x 14 x 7 ~ mm (WxDxH) (Without Bezel)


The dimensions is roughly similar towards the USB specification with the width of \$20mm\$.

In this case this is the height that I have said before.


If it wasn't enough, I could only suggest that you would need to take a deep research and develop a pattern which needs patience.

Offline ways:
If in the nearest place you are right now there is a "USB Dongle" device such as USB flash disk in your home, then you could inspect its dimension and try to plug any other device to make sure they don't interfere each other.

Online ways:
Take a look at online store or specifications web-pages, what dimensions are "USB Dongle" devices usually sold, read the review about its dimension and observe what each customer would like, also at different demography.