What is the meaning of こう how it’s used in this context? And what is the purpose of とか？I know なんか is similar to “like” and っす is short for です so it seems like this is a pretty informal sentence.
こう just means "like this", from このように. He might literally be demonstrating the way he was walking and how he fell as he speaks, but it's more likely that he simply used it as a filler word (in English you might use "like" in this way). As such, I believe it's used here in a similar sense as the "like" in "I was like walking, and then like, I fell, and it like hurt so bad!" (liberal partial translation).
とか means "and such" (here are examples of usage along with some analysis), but here it again may be used simply as a filler word (in English, "n-stuff" might be a relatively close alternative). You could thus attempt to translate it as "So I was walking to the store n'stuff, and then like..." but it's not quite this obnoxious in Japanese, so the translation would not be completely faithful to the nuance.
I would add that the なんか is also just filler and could similarly be translated as "like" or maybe something to the effect of the "I dunno, like" in "I, like, I dunno, like, fell, and (...).".
And as noticed by Chocolate, the っす after なんか is likely a mistranscription, and it more likely refers to the ス in "スベッて転んだ" as the person interrupts his stuttering for a moment by interjecting こう. This would ultimately make the translation of that part go like "I, like, I dunno, f-, like, fe-, fell, and it really hurt!" Something similar may have occurred with the 歩ってて (presumably 歩いてて), although apparently this is also used in some dialect, so it's hard to say whether this was intentional or not.
Related: What does こう in 何ていうかこう mean