Most patterns calling for very small count fabric call for stitching over two threads instead of one (there are some exceptions, so make sure you read carefully!) Linen and evenweave have narrower, more "natural" looking weaving that often isn't completely and totally even, whereas aida, fiddlers cloth etc have very defined "squares" that are made for cross stitching and it is intended that you will stitch over one.
So, when you have a pattern calling for 28 count linen over two, it will stitch to the same finished pattern size on 14 count aida over one. If you want to convert a pattern from a linen count to an aida, and want it to remain the same size, you just have to halve the count - 32 count linen to 16 count aida and so on. Sometimes that won't work exactly, such as with 35 count, but you can get pretty close.