I don't know the smallest but everything I have read or seen recommends a 24 tapestry needle for 14 ct adia or 28 ct evenweave. This is what I have been using and it works out well. Plus the eye seems to be good sized so I can thread it with just a little help from my needle threader. Keep in mind I am just another stitcher and not an employee of ECS.

The size of needle you use depends on the count of your fabric.
Size 18 use for 6 or 7 count Aida
Size 20 use for 8 count Aida
Size 22 use for 11 count Aida and 22, 25, & 27 count Evenweave
Size 24 use for 14 count Aida and 28 count Evenweave
Size 26 use for 16 count Aida, 32 count Evenweave and 22 count Hardanger
Size 28 use for 18 count Aida and 36 & 55 count Evenweave

JM, There is a website that has quite a bit of information about cross stitching in general. One of the things they cover is what size needle to use with what size fabric. If you google: Salt-and-Pepper.com, on their home page scroll down and on the right side you will see "cross stitch helpful hints". Click on that and you will see a good amount of information. I printed this out a long time ago. Hope this helps you!!

I have always used a #24 on 14ct aida but I used a #26 on my last project and I think it was much easier especially when coming back with the second row with 4 strands already in the hole.

I vary. Most of the time I use 24 on 14 count. But, in a pinch I will use 26. One time, my husband was on a business trip and took stitching with me-forgetting my needle. I had a sewing needle with me (emergency) so I used that. Not perfect but like I said in a pinch. I don't recommend it, but it worked..