Once you've gained a little bit of experience with knitting, there is a free pattern archive with lots of cute pet outfits that are fairly easy to resize. There are also quite a few good dogwear knitting books on the market.
A 9" needle length should be sufficient, unless you are making a dress of some sort. Increasing the stitches for a skirt often requires doubling the number of stitches on the needle, and that can get crowded, even for a Chi sized dress. However, for a basic sweater, I find that the 9" length needles are just about perfect, as the longer needles get in my way on smaller projects.
If you've never knitted anything before, however, I do suggest you try something simple first, like a dishcloth, in the same stitch that you will use for your first doggy outfit. You can use a worsted weight cotton for your "practice" dishcloth, then switch to a worsted weight acrylic for your doggy outfit (or you can stick with the worsted weight cotton, if you like the colors). Cotton and acrylic yarns tend to be what I stick with, because they handle washing well, and let's face it, we're putting these outfits on DOGS, lol...and small ones at that, who are close to the ground/dirt.
I like to use Red Heart or Caron Simply Soft yarns (Caron has a lovely silky feel and sheen to it, while Red Heart has a lot of different - and bold - variegated yarns to choose from). These brands are relatively inexpensive (usually they run between $3 and $4 near me), and they handle tough wear. I know there are some absolutely gorgeous yarns at local yarn stores, but I have a hard time paying $10-$20 for an item of clothing that I know my dog is going to roll around in the dirt and grass, and that I am then going to have to hand wash, lol...