Unfortunately it doesn't seem quite easy to get the (possibly different) page sizes (or *Boxes for that matter) inside a PDF with the help of Ghostscript.
But since you asked for other possibilities as well: a rather reliable way to determine the media sizes for each page (and even each one of the embedded {Trim,Media,Crop,Bleed}Boxes) is the commandline tool pdfinfo.exe. This utility is part of the XPDF tools from http://www.foolabs.com/xpdf/download.html . You can run the tool with the "-box" parameter and tell it with "-f 3" to start at page 3 and with "-l 8" to stop processing at page 8.
Example output:
C:downloads>pdfinfo -box -f 1 -l 3 _IXUS_850IS_ADVCUG_EN.pdf
Creator: FrameMaker 6.0
Producer: Acrobat Distiller 5.0.5 (Windows)
CreationDate: 08/17/06 16:43:06
ModDate: 08/22/06 12:20:24
Tagged: no
Pages: 146
Encrypted: no
Page 1 size: 419.535 x 297.644 pts
Page 2 size: 297.646 x 419.524 pts
Page 3 size: 297.646 x 419.524 pts
Page 1 MediaBox: 0.00 0.00 595.00 842.00
Page 1 CropBox: 87.25 430.36 506.79 728.00
Page 1 BleedBox: 87.25 430.36 506.79 728.00
Page 1 TrimBox: 87.25 430.36 506.79 728.00
Page 1 ArtBox: 87.25 430.36 506.79 728.00
Page 2 MediaBox: 0.00 0.00 595.00 842.00
Page 2 CropBox: 148.17 210.76 445.81 630.28
Page 2 BleedBox: 148.17 210.76 445.81 630.28
Page 2 TrimBox: 148.17 210.76 445.81 630.28
Page 2 ArtBox: 148.17 210.76 445.81 630.28
Page 3 MediaBox: 0.00 0.00 595.00 842.00
Page 3 CropBox: 148.17 210.76 445.81 630.28
Page 3 BleedBox: 148.17 210.76 445.81 630.28
Page 3 TrimBox: 148.17 210.76 445.81 630.28
Page 3 ArtBox: 148.17 210.76 445.81 630.28
File size: 6888764 bytes
Optimized: yes
PDF version: 1.4
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