How to set a specific size for an object in a Photoshop image

If you are looking for automation then I am affraid there is no easy way to automate that task. The only way that comes to my mind, manual way, is to use Smart Object layer > Replace Contents to quickly replace content on layer then to manually transform according to guides on screen. You can turn on modal control for Replace Contents and Transform steps to choose which file to use for replacement and later how much to scale or transform image.

Further thinking you may be able to automate this process with manual transformation. It is rather complicated batch process for someone who is not familiar with actions and batch processes.

Action that should be recorded must run during batch process to copy open image then to paste it in another open document where it should be manually scaled to measurement which can be set up in document for that purpose (always open document during batch, open document before even start batch) then copied again and pasted in previous document (open image in batch) and lastly to change image dimensions to desired final output dimensions. I have dome similar things in the past but this time I am writing from experience, haven't checked anything in application.

You can use the ruler tool to measure how long the book is, the just calculate the scale and transform to the size you want.