You can't really make a shirt longer or pieces wider. Usually there's only half an inch or so of fabric in each garment's seam for letting it out and sometimes less. So you'd have about a quarter of an inch of fabric to work with at any given seam. This can help in jackets, shirts and skirts that have panels and if you're lucky pants but you would have to tack on extra fabric which can come out really badly if it's mismatched in terrible ways. It is very unlikely that the hem at the bottom of any shirt will allow for a real change in length.
The fabric does matter. The bias to line up, the nap has to work, the material has to be similar (jersey knit vs corduroy vs muslin or whatever). The thread also matters but less so, mostly that's picking color and all purpose thread is probably fine. The stitch and tension matters more than the thread itself.
If you want to change the garment entirely into a new garment that can be done. If you want the same shirt just bigger that will probably not work out very well. I suggest you head to a bookshop and find a magazine called Altered Couture
which could give you ideas on creating new garments from those shirts.
The Vogue Sewing
book is awesome.http://www.amazon.com/Sewing-Revised-Updated-Knitting-Magazine/dp/1933027002/ref=sr_1_1?ie=UTF8&qid=1293690206&sr=8-1
It gives good explanations accompanied by pictures and really goes into garment construction. Altering a garment is almost ALWAYS harder than just making a new one. The only thing I'd say is easier is hemming things because you can always make something shorter. Changing the actual fit of it is really difficult and if you want to do that I suggest starting with things that are at least two sizes larger than you wear so that you have the room to go down. Lengthening is basically non-negotiable with clothing.