The final resolution is determined by a few things.

Original resolution – This will be the most important factor. If you have a video that looks clear and fine on its own but has a low resolution, and you add this to a project that has higher resolution video already loaded, then you are expanding in on this lower res file; this will cause pixilation to the lower res file.

Ratio – This is part of the resolution. This is meaning that if you have a video that is 480 x 360 and you view it at larger screen size, you will cause pixilation to the video. Basically the same as I mentioned above. But just as a general playback setting.

Export 1 – If you export your project in a ratio higher than the original content will cause pixilation.

Export 2 – If you export your low resolution material into a low resolution file format with lower quality settings, then you can get unwanted poor resolution.

It is very important to know what resolution your original material is in as well as exporting in a resolution that is supportive of the material. If you constantly alter the format of the video, you will also degrade its resolution.
 

You cannot fix the file you have created, you would have to determine first what caused the pixilation. Was it that you exported with low quality or used a high compression format such as QuickTime or WMV with a lower quality setting. Was the original materail just in a low resolution and you exported it out in a higher resolution, therefore it suffered pixilation.

I hope this helps