what is the USB cable Length ?

About 4' to 6' feet. but with the power plugged in you can have long cables going into the hub.

Is the StarTech.com ST4300USB3 hub compatible with older USB devices?

Yes, the StarTech.com ST4300USB3 hub is backward compatible with USB 2.0 and USB 1.x standards, allowing it to support older USB devices.

The dimensions of the StarTech.com ST4300USB3 hub are 0.91 inches in height, 2.68 inches in width, and 3.74 inches in length. It weighs 1.94 ounces.

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StarTech 4-Port USB 3.0 Hub, High-Speed USB Expansion Adapter, Plug & Play, Portable USB Hub for Laptop & Desktop

Add four external SuperSpeed USB 3.0 ports to your laptop Ultrabook or desktop from a single USB 3.0 connection
Works with virtually any USB 3.0 equipped computer
Ideal for Ultrabook & MacBook users with limited USB ports
Compact form factor / Includes optional power adapter
Add high speed USB 3.0 ports to your PC / Connect peripherals and external hard drives or flash drives in the home or office
4 Port USB 3.0 Hub / Notebook & Laptop USB 3.0 Hub
Desktop USB3 Hub / External USB Hub