For mechanically drilled holes, stay above 8 mils.
Laser-drilled holes (microvias) can be smaller diameter.
(12 mils holes are preferred if your design can accommodate them)
sorry you had to wait so long for an answer. (I'm not a PADS user)
In the future, for PCB design related questions you might try joining the IPC Designer's Council Forum.
You can get info about IPC Forums at ipc.org
With Apect Ratio 1:8 you can stay safe. Bigger is better :-)