Likely your sequence settings don't match your footage settings. You'll want to set your sequence settings to be the same as your footage, or you can scale your footage down to match your sequence. The easiest way to match footage to sequence frame size and frame rate is to simply drag your footage into the timeline area when it has nothing in it. Premiere will make a sequence that matches your footage.
What Graeme Bull said is (likely) your issue. His advice is probably going to address the issue. I will add that you can right clip on your clip (or group of clips if selected) and select create sequence from clip and it will create a matched sequence for you. This (I thjnk) is a slightly easier, but you can make the call after trying it.