The easiest way to accomplish this is to use the crop tool. On the crop tool's option bar enter the H&W and resolution in px/in. Note that for printing it is desirable to have the resolution in the 240-300 px/in range, although I have printed successfully at a lower value. For web work, 72 px/in range.
On your display I don't see evidence of pixellation. but I'm looking at a monitor.
Also, suggest that you make the resolution of the canvas the same value as that of the individual picture files.