ok so i testing my 3d printer with 20mm cube which my printer printed it fine but when i use something different or bigger like 40mm cube etc or more object it always prints in a same size that is of 20mm cube so whats the wrong???

Is your slicer set to auto-scale objects to fit the bed?

You should be able to see in your slicer the size of objects. Do they look right there?

i am using slic3r 1.1.7 and did not found any auto scale option in my slic3r so where it is????

again i printed a 42 mm square but my printer prints only 15mm of square???and i dont know where to get original size??

I don't know about the scaling problem, but why are you using an ancient version of slic3r? Get the latest developer version 1.3.0-dev. It fixes many bugs in even the latest stable release (1.2.9).

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

could it be your calling slicer from another program like pronterface? and not running it manualy?

There used to be an issue if you set up slicer wrong, proterface would call slicer, it would fail but the original gcode file was still there... so pronterface got the orriginal gcode, every time.

try a completely different object, say a cylinder, verify it really is looking at the new file as I suspect its not a scaling issue

ok got the solution i just changed the bed size and centered the x,y axis and problem solved

previously my x,y is at 0,0 position that is why printer is printing wrong..

AND got new PROBLEM which is when i print 40mm object the finel print result output is 33mm how and why 7mm difference?????????????????????????????????

@robobuilder
how many times you read this Triffid guide , well be your axis step are off. or you got some really slack belts!