Question 1

what the storage size?

Accepted Answer

I bought the drive enclosure.  It did not have a drive in it.  If you wish you can buy one with the drive but it will be much more expensive.  I bought a 1TB WD 7200 rpm drive.  It is incredibly easy to install and set up.  The directions are easy to follow.  If you have woked on a PC, it's a piece of cake.
If you buy the enclosure, you can put any capacity 2.5" Sata drive in it.  I recommend a 7200 rpm or higher drive.  The slower 5400 rpm drives take a lot longer to read/write.  In my opinion it is worth the few dollars more for the faster drive.  Once you install the drive you will have to format it.  I recommend using only one partition for the whole drive.  You can find a 7200 rpm 1TB drive for under $80 at newegg.

Question 2

What is the max drive size ssd this will support?

Accepted Answer

Overview says tested with up to 1 TB.   I wouldn't  exceed suggested size to find out the hard way.
 I tried with 500 G and 750 but haven't audited whether data is actually bit for bit there.  I have heard stories of usb thumb drives that have been hacked to report Much more  capacity than they have and when you write too much, it overwrites older data.
Greedy seller takes tiny flash drive and sells it as huge flash drive.

Brand	StarTech.com
Model	S2510BU3PWPS

Color	Black
Type	others
Material	Plastic
Internal Interface	SATA
External Interface	USB 3.0

Dimensions	Height:0.6" x Width:5.3" x Depth:3.1"
Weight	2.65 oz

StarTech.com 2.5in USB 3.0 Encrypted External Hard Drive Enclosure - Portable SATA HDD

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Item#: N82E16817707308