There are several good Photo editors. Photoshop CS, Photoshop Elements 2 or
Elements 3, and Paint Shop Pro.
There is a learning curve to do what you want to do with any of the Photo
Editors.

File size depends on the file format and the size of the final image.

An A3 print at 300 DPI would require a 4960 x 3507  pixel image.
Assume no margins. Printer margins will make the printed area smaller.

From http://www.scantips.com/calc.html
4960x3507 pixels will print 16.54 x 11.69 inches (420.1 x 297.0 mm)
at a MAXIMUM of 300 pixels per inch (118 pixels per cm)
(Maximum meaning any cropping lowers printing resolution)
4960 pixels / 300 ppi = 16.54 inches
3507 pixels / 300 ppi = 11.69 inches

This 4960 x 3507 pixel image size is 17.39 million pixels, with Aspect Ratio
1.41 : 1

The image size in memory is:    (see explanation of size and megabytes)
99.533 MB if 48 bit RGB
66.356 MB if 32 bit CMYK
49.767 MB if 24 bit RGB (most common)
33.178 MB if 16 bit GrayScale
16.589 MB if 8 bit GrayScale
2.074 MB if 1 bit Line Art

The above values are about what a uncompressed TIFF file would be.

JPEG would be much smaller (around 10%-50% of the file sizes) JPEG file size
depends on the amount of compression and the image content.

I find it easier to do this job in a page-layout program, such as Microsoft Publisher.
Position the photos as you want them, then link them together.  Cut and paste into an
image-editing program, and save as a .jpg file.