@markusschaber: not at the moment, but soon (ish) it should now that .NET Core 2.0 is out. I've updated MonoMac to work on it, and it should also be able to work on linux using @cra0zy's native gtk port of Eto which removes the gtk# dependency.
As for windows, there's no UI framework that works on .net core.
@philstopford: You shouldn't be setting the size of elements that change size based on the platform.. I can't remember, are you using a PixelLayout for you layout? That would also be a problem when trying to size controls on different platforms or between Win7 and Win10.
e.g. a TextBox you'd set the width, but not the height
so that the platform can decide what the ideal height is