Name the boundaries
The sheet size is the physical piece of paper. The printable area is the region the device can mark with the selected paper and mode. The image rectangle is the physical photo size you requested. A photo border is extra space deliberately included around that image.
For repeated prints, the gaps between neighbouring frames are another measurement. A gap is not an outer margin, and neither should be confused with the empty area created when a photo is fitted inside a frame of a different ratio.
An 8 × 10 photo on A4
An 8 × 10 inch portrait photo measures 203.2 × 254 mm. A4 is 210 × 297 mm. Centring the image leaves 3.4 mm at each side and 21.5 mm above and below. The large vertical spaces do not compensate for a side margin that is too narrow for the device.
If a printer requires more than the available side space for the selected setup, the layout cannot be printed as drawn at actual size. A Fit option may reduce the entire page to avoid clipping, but that also makes the eight-inch image narrower.
The same image on Letter
Letter is 8.5 × 11 inches. A centred portrait 8 × 10 image leaves 0.25 inch at each side and 0.5 inch vertically. In millimetres, those spaces are 6.35 and 12.7. Compared with A4, the side allowance is larger and the top and bottom allowance is smaller.
This comparison is geometry, not a recommendation that Letter will work with every printer. Check the actual printable-area instructions for the device, media type, and feed arrangement. Manufacturer specifications can differ across modes even on one model.
Treat borderless as a specific device mode
Borderless support depends on the printer and the selected media settings. Do not infer it from a zero-margin design in the tool. Read the device instructions for supported sheet sizes, edge behaviour, and any scaling or extension performed by that mode.
If preserving an exact crop is important, inspect a physical test and the driver’s preview. A frame lip or later trimming can also obscure the edge after printing. Keep essential detail clear of those boundaries when you know they will be present.
Build the layout around known constraints
Set the paper size first, then reserve the margins you can actually use. Place the photo at its required dimensions, add any repeated copies and gaps, and check that every rectangle remains inside the usable region. If it does not fit, change a deliberate design choice rather than hiding the conflict.
Export a physical layout PDF, match the paper in the print dialog, and use actual-size output. Measure a calibration mark and the first image before trimming several copies. The preview gives you exact planned geometry; the first measured print confirms how the device executes it.