Okay, the size of chuck, 1/4", 3/8", 1/2", etc. refers to the capacity or largest size of drill bit you can fit in the chuck. You have to be careful when selecting a chuck as many have a "minimum" size as well (some also go down to "0"
). Basically it will depend on what size of holes you are planning on making.
As you have seen the chucks also come with either integral shanks or are marked as having a (0,1, 2, etc) JT (or "J") -Jacobs taper. The Jacobs taper refers to the tapered hole in the backside of the chuck (or on the shank) so, in your case, you'd want a shank that is 1 MT to fit your tailstock. On the other end of the shank you'll either have an integral chuck or a Jacobs taper to match to whichever chuck you select.
For example: On this page,
http://www.use-enco.com/CGI/INPDFF?PMPAGE=545&PMCTLG=00 you can get a 1 MT shank with say a 6 JT on the other end Model #240-2718. Now, go to this page,
http://www.use-enco.com/CGI/INPDFF?PMPAGE=550&PMCTLG=00 and we see a Jacobs 6 JT chuck, #290-1330 that will fit on the end of the shank.
These shanks and chucks are just used for an example, this should not be taken as a recommendation as to what you should actually purchase. That has to be decided by you and what would fit your needs.
Hope that helps though,
-Ron