In the last beta: https://dl.dropboxusercontent.com/u/13064071/plantuml.jar
You can have :
scale max 50 height
There is an additional setting: you can define a environment variable PLANTUML_LIMIT_SIZE (which default value is 4096).
You have to define this env variable before launching PlantUML.
setenv PLANTUML_LIMIT_SIZE 100
Another way is an option in the command line:
java -DPLANTUML_LIMIT_SIZE=70 -jar c:/ant/lib/plantuml.jar -v foo1.txt
This has not been highly tested, so feedback is welcome.