Well, it is certainly the size you asked for - 25.4 x 95.6mm when I import the STL into SU.
And that seems to be a pretty exact conversion.
[EDIT] Not quite exact - to more decimal places, 3 49/64" is 95.646875mm. But that’s what you’ve got to the nearest 1/1000th mm. See updated picture with more digits in Precision.
You don’t ‘set’ the ‘dimensions’ to 1/64".
You might have set the Units Precision to 1/64", and perhaps turned on Length snapping.
But that only sets the precision with which SU DISPLAYS the dimensions. And if you are using Length Snapping it may force your line, rectangle or other geometry to slip off an accurate inference to existing geometry.
If you want accurate geometry slightly more easily, turn off length snapping, type in your exact dimensions as you draw, and watch out if the Dimension tool or Tape Measure shows a tilde ~ in front of any dimension. That shows it is an approximate dimension, not exactly a precise fraction of an inch, or mm, to the precision you have set.
BUT, that doesn’t explain why it isn’t printing the size you have drawn.
It’s a very simple shape. Why not set Window/Model Info/Units to mm, precision to 0.1mm, Length snapping off, and type in the dimensions in mm as you draw two rectangles and a PushPull operation.
Try printing that.
If it is still off-size, the error isn’t in SU, but in the slicer software, or the difference between the path of the centreline of the print head and the edges of the extrusion if it is a Filament Deposition Printer - about half the diameter of the nozzle, times two again for opposite sides of the rectangle.
I find my 3D printer produces objects about one nozzle diameter too large outside, and holes too small inside.