Sometimes when importing a PDF of varying page sizes into Photoshop, smaller pages are increased in size to match one of the dimensions of the larger pages.

Is there any way around this?

I experienced the same behavior, and it just appears to be a glitch with the import (Photoshop doesn't support multiple sizes through batches very well - and the import dialogue is basically a built-in batch action). It sounds like you are referring to a multiple-page PDF (which is where I have also found the issue). The only real work-around I have found is to split your PDF and bring the pages (or groups if you have some same sizes) into Photoshop separately.

- How exactly do you proceed and are those pdfs that combine pages of various sizes?

I export a PDF with multiple pages of varying size from InDesign CS5.5 and then open it in Photoshop CS 5.1. In the import dialog I select all of the pages and then set the dpi to 72. When all of the documents are created some of the pages are scaled up/down from their original size.

- Why do you convert the pdfs and are you aware of the quality-loss this incurs with regard to vector and type data?

I convert PDFs to JPEG for use in Flash. I am aware of the quality loss and the difference between vector based images and bitmap images but the JPEGS exported from InDesign are of a lower quality than the JPEGS produced from Photoshop when re-saving the PDF pages.

- one might be able to use a Script that opens the pages individually or creates new documents for each page, places it and clips to its format after placing.

I am very interested in this. I will look into writing a script that will solve this issue. I was just surprised that Photoshop had a problem importing the PDFs.

Thanks for all your responses.