I set the size of a .png in one piece of software. When I bring it into Suite it is many time larger. Changing the dpi changes how large it becomes but why doesn’t it stay the same? And I’m setting the size in mm.
And it gives the dimension in a green box above the graphic and always says 26%. 26% of what?
Context is key here…
- Where are you seeing this?
Provide screenshots to explain.
PNG can be measured in many different dimensions. When using on screen, pixels are the most common; when aiming for traditional 2D print, mm are used (unless you are American, then likely in hands of a horse 
).
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You may not understand what DPI is used for.
- DPI (dots per inch) is a print-related term for traditional 2D printing
- PPI (pixels per inch) is a display-related term for screen-based images
- Both determine how much detail is used within a set size. A 150DPI image has four times less information than a 300DPI image, when printing the same square region.
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Percentage shown, this is a guess as you have not provided where you see this.
The 26% is possibly a scaled value, the scale that the software has applied to show it in the region where the image is shown. Stretch the region the picture sits in, and the scale may well change.
If you are getting warnings that your image is too large, scale it down in the software you used to create it.
If you add context and examples, I or others may be able to help you more.