I've created a model in Sketchup and exported the file as a .stl Since the model is 80cm x 24 cm i need to cut it in meshmixer. But when i open it in meshmixer it appears tiny. Why doesn't it open as the real size?
You mention your dimensions in cm. Any chance that the STL export is also in cm where Meshmixer might expect mm?
Assuming your Sketchup template was set to cm:
To validate this guess you could measure/eyeball a known dimension and if that comes out to be 10 times too small you could scale your model by 10 times. Alternatively you could adjust the template in Sketchup.
Additional information on STL files: By it's very nature, the STL file format is just a bunch of unitless numbers in a well defined structure. This structure represents a set of points (vertices), lines (edges) and and triangular surfaces (facet). A few simple rules apply to exactly how all this should be defined. (See here)
But the key thing here is that an STL file is not aware of units, the program used to generate the STL file (e.g. CAD software) needs to be told what units to use, and accordingly the reading program (e.g. slicer) needs to use the same settings. Generally in mechanics applications we default to mm, at least in metric land. All this is explained a bit more verbose here.