Generally you dont want to be taking crazy odds like -10000. Its just not going to be worth the risk in the long run. In saying that it really depends on your bankroll, the sport, the specific game, and situation. If you can consistently beat the closing line by a big enough margin, you should be profitable long term. That is a skill that takes a lot of practice/ trail and error with your betting strategies.
For size of your wager, a good way to start would be just to bet to win a unit on each game. So if you like a fav at -250, and your unit size is $100, you would just bet $250 to profit $100. With a dog, you would just bet $100 to win the bet.
Sorry if that didnt really answer your question, but thats my opinion on it anyways. GL!