Photoshop help desperately needed please

[SellingDilemma]

I have a load of handprints that family members have done - 1 handprint per a4 sheet. I have photos of these from above with the whole sheet in the frame. I requested them like this so I can use the a4 size as my scale. I need to get the handprints and keep them at the correct size then compose them onto a single a3 page. I am a novice to photoshop, so I can select them and change colours etc, but I have no idea how to figure out the scale to make sure they are the right size. Can anyone help me figure this out please??

They will stay in their correct size if all the original files are at the same dpi. If you hold down control alt and the letter i you will see size and resolution. I've posted an example below. If all the resolutions are the same, then all you need to do is to make a new file with the same resolution and copy them in. Hope this helps.