galaxy s2 charging cable doesn't fit galaxy tab 3 They both have micro usb ports but i thought I could use my current galaxy s2 cables with the tablet and they don't fit.  Are there different sizes and how can you tell when getting a cable?

Thanks
It should easily go in. The ports on the two devices are the same and with same orientation of the port as well. Maybe there is something stuck inside the port on your Galaxy Tab 3. You can usually tell by the shape of the male port. The two slanted ridges on the size are an identifying feature of Micro USB A/B. B also has two little latches on the bottom. Micro USB standards A and B are compliant with each other as well but these two devices should have the exact same Micro USB B Port.