LR and PS can certainly do this, but you will have to do a bit of arithmetic to get things set up correctly.
Here's an example: Suppose you want to print on 5" x 7" paper and you want a half inch border all the way around the print. This means that the image itself has to be (5.0 - 0.5 - 0.5)" by (7.0 - 0.5 - 0.5)", or 4" x 6". If your image is any other size than 4x6, it will have to be modified to become 4x6.
If the image has the same aspect ratio, ie, 1 : 1.5, for example, if it's 2x3" or 8x12", the problem is simple: You just scale the image equally in both directions so that it becomes 4x6.
OTOH, if your image has a different aspect ratio from what is needed to print correctly, the procedure becomes a bit more involved. First, you have to realize that in 99% of the cases, you will not be able to simply scale the image differently in the horizontal and vertical directions to make it become 1 : 1.5. Such a simple approach usually only works if the original aspect ratio is within a percent or two of 1 : 1.5. If the difference is much larger, changes in the shapes of objects (especially people) usually become noticeable/objectionable.
In cases where the change in aspect ratio is larger, you will have to either reduce the excess in one direction or add some material in the other direction. This can be done by techniques such as cropping, stretching relatively unimportant edges of the image, or using a content-aware scaling tool.
This procedure isn't rocket science, but one has to think about what one is doing and do the necessary arithmetic to see exactly what needs to be done before you attempt to make changes.
HTH,
Tom M
