Hello,
no there is no limitation in the trial version and there should be no problems with 256x256 images or with background. If you are using the default settings, the background should be transparent. If it is not, look closely at the "Transparency" option in the "Image from Icon" wizard. It has several options how to determine what should be transparent. BTW if you are creating 256x256 versions, you should increase the size in the Rendered Image editor in the "Image Properties" panel to 1024x1024 or so to have good enough source image for icon conversion.
One more thing: do not trust what you see in Explorer while viewing and .ico file. It has its own way of picking the image to display. If you on the other hand for example customize a folder with and icon, the 256x256 image will be used without problems.