Me on a personal note: I stay away from Alterna...they were in that "comicsgate" mess some time back.
Basic rule- comic size is 7" x 10.82"(don't know what that translates to size wise in pixels)...color/resolution you wanna keep it at 300 dpi or higher(NO higher than 600 dpi though).
The 2 companies I've used to print my comics- Ka-Blam and Greko Printing(also known as Comix Well Spring). I love the interior stock that Ka-Blam prints on; their covers are pretty much that same type of stock, so nothing to bally-hoo about...Greko's price is a smidge cheaper than Ka-Blam; their basic interior stock isn't anything to write home about, but the cover stock- WOW!
I print my webcomic chapters through Greko to sell at conventions; I get a more glossier stock- which costs me a little more but I don't mind- I treat the printed version of my webcomic like a premium book...accompanied with the awesome cover stock that Greko does(and my page count), I get to sell the book for more than my other title- which I print through Ka-Blam.
Here's a few pics of my printed webcomic through Greko:
My other title printed through Ka-Blam:
Ka-Blam has a downloadable template in which you can use to format/size your pages- I use it on Photoshop to make sure my pages are correctly formatted before I send to print.
http://ka-blam.com/main/
Also also- companies like Ka-Blam & Greko are referred to as "POD"(print on demand) or digital printing(done by digital printing copiers). You can order as many as you need/like- from as little as 5 books to as many as 200. This method of printing differs from [traditional] offset printing(which is printed through large machines); the minimum amount you can order with offset printing is 1000(coz it costs that much to set up the machines and print plates).