Check your blade tension. Pull the back of blade and let go when adjusting. You should hear a pinging noise indicating enough tension. As for blade size it all depends thickness and hardness of wood being cut. Guide your wood through and let the blade do the cutting. Sometimes blade clamping may be an issue. Other times quality of blade may be an issue.
Two good quality brands of blades are the Flying Dutchman and Olson. Myself I've only used the Olson brand.