If you have been playing with blender you know the files can get fairly large quick. For example, the model I used to create the following pattern had over 2 million verts and over the 100 meg limit to import the stl into designer.
A way to work around the limit is to create the model as different parts. This model was split into 4 different objects. The 2 wings, body, and tail. They can be imported separately by using export STL and use selection only button and selecting each piece individually. They can then be merged in designer.
This model was created using only 4 different feathers that I copied, rotated, and resized to get the varying plumage but because the feathers were fairly detailed the file size got large quick. In the end I created several hundred feathers to create this model.