Bethesda has announced that starting with this week they are lifting up the 100MB restriction for the mods, so that you can now upload mods without thinking about their size.
Before this feature will be made available for everyone, both Skyrim’s PC launcher and Creation Kit will be in beta tests. In the source link you’ll find the steps that you have to take in order to participate in these beta, and below you can find details about the Skyrim launcher and the Creation Kit 1.9.33 Update:
Creation Kit 1.9.33 Update
- Creation Kit now supports unlimited file sizes for mods on the Steam Workshop.
- The Preview Image is limited to no larger than 1 MB.
- The Skyrim Launcher no longer downloads mods from Steam Workshop. Downloads are now managed by the Steam Client. The Skyrim Launcher updates the game after the Steam Client finishes the download.
Source: Bethesda Blog