OK… disclaimer here, I’m not entirely sure that I understand your question (I don’t use PhotoShop or Design Studio) but… assuming that I do understand, I think the problem you’re experiencing may actually be geometry and not the size of the invitation.
If you create a frame that is 5x7 inches, and then shrink it slightly on a 5x7 inch card, you’re NOT going to end up with equal space on all sides of your frame - which is crazy and counterintuitive, but if you want to maintain an equal margin on all sides of your frame, you actually need a different aspect ratio than the card itself.
So, for example, let’s say you have a 5x7 card and you want a frame with a one-inch border between the frame and the edge of the card. In order to achieve that, you need a frame that’s 3x5 inches (an inch off the top, an inch off the bottom, and inch off the right, and an inch off the left.) BUT, if you shrink a 5x7 inch frame so that it’s 3 inches wide, you end up with a frame that’s only 4.2 inches high! (because 3/5 of 7 = 4.2) So, you actually need a taller narrower frame than the size of the invitation.
I spent an entire afternoon bashing my head against this problem trying to come up with a frame that would look centered on a business card. If you look at this design (Simple Dusty Blue & Lilac Watercolor Wash Feminine Business Card | Zazzle) the frame is centered in a 3.5"x2" card. But if you open the design tool and grow the frame, you can see that it’s actually a wider and shorter aspect ratio than the card!
So, I think the formula for getting the frame right is to subtract two times the space you want as a border between the frame and the edge from each dimension of your card size. So, if you want an inch border between the frame and the edge of the card (like our example above) you need to subtract double that amount from each dimension - so you need a 3x5 inch frame. For a half inch border, you’d need to subtract an inch from both the height and the width of the card, giving you a frame that’s 4x6 inches. If you want a quarter inch border, then you need to subtract half an inch from each dimension, giving you a frame that’s 4.5x6.5 inches. So the aspect ratio changes depending on how big you want the border to be. Hope that makes sense!
All that being said… Zazzle actually discourages designs that use frames because the way the print process works, they can’t guarantee that it will be exactly centered even if your design is perfect. You can see Scott’s explanation here: https://community.zazzle.com/t5/general-zazzle-discussion/feedback-from-samples-i-ordered/m-p/23337/highlight/true#M3265
ANYHOW, I hope that helps… of course it’s perfectly plausible that I’ve completely misunderstood the problem, in which case you can just ignore all of my blathering! Just hoping to save someone an afternoon of headaches because it is VERY counterintuitive… at least for the way my brain works! 😋