how do merge 2 clips and keep the same size ?

i just started using quicktime pro. i need to combine 5 different videos into one file. i have figured out how to merge these, but some are different sizes. the first video will be in full size, then when the next one starts its in a smaller one. it will be only in the corner of the larger video. have worked on this for a long time time now. has me stumped !!

You have a number of options here. If all have the same aspect ratio, you can either scale the smaller clips upward to match the larger clips or do the opposite and scale the larger clips downward to match the smaller ones. If the aspect ratios are different, you can still apply scaling but this will add distortion if you "fill" the frame. If you don't want the distortion you can partially scale and/or apply an offset to center the smaller framed clips within the larger display window with or without a colored or patterned background and/or a custom graphic frame if desired. Another option would be to "mask" larger clips to "crop" the scaled or unscaled content to the same size and/or shape as the smaller. In short, your options are basically only limited by your imagination.