The SharpDevelop license is LGPL, but that only applies to redistributing SharpDevelop or its components (e.g. our text editor or C# parser).
There are no restrictions on the use of SharpDevelop.
We use SharpDevelop to work on SharpDevelop, so it can certainly handle large projects. C# code editing is rock solid in my experience; but some of the newer or rarely-used features (WPF designer, Class Diagrams) not so much.