I've got some mockups of laptops to display my work. All of them are large files but I wanted to use it in thumbnails. The problems it's when I change the psb to my work and then make the psd file into the size of the thumbnail the work is completely distorted. I know there will be some distortion but I have a feeling it would look better if the psb was the same size as the placeholder on the original psd.
Unfortunately your question is slightly unclear, so I'm going to try to clarify your question, and potentially answer it.
I've got some mockups of laptops to display my work. All of them are large files but I wanted to use it in thumbnails.
Ok, as an example let's put dimensions in this - say your mockups are .psb files whose dimensions are 1,000px by 1,000px, like the image below.
The problems it's when I change the psb to my work and then make the psd file into the size of the thumbnail the work is completely distorted.
This part is what's a little confusing. Saving the PSB as a PSD is fine - it may exceed the maximum dimensions of a PSD, but it sounds like you're resizing it. The real question is, what kind of distortion are you seeing?
Say you took a 1000x1000 image and resized it to 500x500 - because it's an even, 2:1 size difference, you shouldn't see any "distortion" of the form of stretching. If, however, you had small details in the original image, or perhaps text, you may lose that detail, or the text may become harder to read. This is expected because the image is now half the size.
On the other hand, say you took a 1000x1000 image and resized it to 800x500 - this is not the same aspect ratio, and therefore you have to make a decision. You could resize the image to 500x500 but put it in an 800x500 container, so that it would need a background of some kind (or perhaps transparency).
You could resize it to 800x800, which would cut out some of the image.
You could resize it to 800x500, but because of the different aspect ratio, you would see some distortion.
So the real question is: how, exactly, are you resizing the images?
Huh! Smart filters in mockups often expect exactly certain size smart object images. If you put there something smaller than the placeholder image the result can be full crap. That's sure if the filters use something geometry affecting like warps, blurs or there's layer styles which contain pixel dimensions. Or there's used blends with some in the mockup given layers.
Make the thumbnail by flattening a copy of your full size properly filled mockup. Then reduce the pixel dimensions of the flattened image.