Yep 15*15 is the UV units I use, you can do what ever suits the runway, apron etc. Runway and taxiway textures are normally a repeatable map, I use the default ones as they are well made. What I have done is to add a 1 pixel line around the edge to give the impression of a boundary, just erase bits of this line to make it look better so its not so in your face.
Adding decals and grunge as I call it are 2 different things. Decals for taxi lines, bay markings etc are added as per the tutorials, it works very well. I also add another decal which are priority 8 and 9, the highest so last applied, which is the grunge layers. This can include tyre markings, oil spills, tarmac repairs, concrete colour changes etc. I do however apply the tyre markings as a separate decal and map for better placement and more detail, This way you can do 2 tyre markings for individually ends if you wish. As these are normally a single bitmap for a large are they do look blurred in FS2, hence the reason for adding other decals as per the tutorials, its great for grunge not taxi lines etc
Background image is normally the fset image at a high zoom, do remember if you plan to upload the airport to the website you will need to remove this image from your project as IPACS will not allow it. If you have followed the instructions about vertex colouring the background image will not show anyway.
Not sure if FS2 uses mipmapping on ground textures..one for the developers to answer
Best tutorial is the written one for max
There is also a post by Antoine about things appearing misplaced in FS2 if you wonder why this may happen to you
Steve