An important part of creating professional looking photo collages is the proper image arrangement and size adjustment. One program that can help you resize the pictures without quality loss is BenVista PhotoZoom Pro.
As its name says, the application addresses to professional users; therefore, you need some experience in image editing in order to comprehend the size adjusting settings.
However, the first time you launch the program, it will recommend you to complete a few steps in order to optimize its performance. This process might take a while, but it's definitely worth the wait.
You will be able to add entire folders of pictures to the processing list, choose from a wide variety of resizing methods and enter the exact dimensions you wish for your photos. To be honest, I would've preferred to get access to some visual effects, like smooth, dilate, contrast, etc.
This application can also be used for converting your photos to other popular image formats. Once you resize your picture, you can save it to various image formats, from JPEG and BMP to TIFF and TGA.
The program managed to resize my pictures without damaging their quality, but taking into consideration its high price and lack of editing tools, I guess it's up to you to decide if it's worth trying out.