Cross-stitches slip between the threads of even-weave fabrics, not through them so the correct needle is important. The wrong size needle can distort the holes in the fabric.
If you are using 11 ct. aida you would use 3 strands of floss and size 24 needle.
14 ct. aida 2 strands size 24 or 26 needle
18 ct. aida 1 or 2 strands size 24 or 26 needle
Hardanger 22 ct. to 25 ct. 1 count size 24 or 26 needle
pictured 14 count (stitches per inch) aida cloth
Aida is an even weave, interlocking cotton cloth. It's available in many colours and comes primarily in 11, 14, 16, 18 count. 14 count is the most often used aida for cross-stitchers.
(pictured) Linen even weave
Linen is a single thread fabric with the completed cross stitch usually covering two threads. On 28 ct. linen, the cross-stitch over 2 holes is approximately the same size as 1-14 ct. aida cross-stitch.
Hardanger looks like linen but is an overlock fabric like aida. it's usually available in 22 ct. and 25 ct. Because the count is so high, the stitches are smaller. They are half the size of a 14 ct. stitch on aida cloth, so are very popular for stitching greeting cards, christmas ornaments and landscape designs.

Creating cross-stitches
Here's another good reference for Needle Know-how
Eight Steps for Choosing the Right Needle!A Guide for Needle Artists by Catherine Bracken
http://www.discountneedlework.com/articles/Needles.htm