Hi Helma,
Welcome to the Sweet Home forum.
The easiest way (if you already know the dimensions required) is to enter either a height, depth or width while importing the furniture, entering any value in this section will change all other values proportionally.
Enter either a height, depth or width
Once the furniture or model is already imported, use the Modify furniture editor but you will need to work out what values to enter in each box, to keep the correct object proportions. (I have requested a Constrain Proportions tick box for the modify furniture window but until then use the methods above)
Modify furniture window
Or
Just thought of another idea!
Another way is to use the new Furniture Library Editor version 1.0 tool (no install needed, just double click to run) import your model, set the size you want, then save and import into Sweet Home.
db4tech
