Memory and Undo options
The Memory and Undo pane of the Options dialog box helps you limit the amount of random access memory (RAM) that Microsoft Expression Design uses when it is running.
Frozen layer memory limit Freezing a layer turns a layer's objects into a bitmap at a resolution of your choice (rasterizes them). However, if you select a high resolution, freezing can be memory-intensive. You can use this option to specify an upper limit of the amount of memory you are prepared to use for rasterizing each layer.
Undo levels (after restart) The default number of undo levels (the number of states Expression Design remembers) is 40, but you can change it to any number from 2 to 200.
You must exit and restart Expression Design for this change to take effect.