Are you using the Linux version? The syntax highlighting files are stored in the mape-syntax folder, and it is searched for in these directories: ~/.mape-syntax, ./mape-syntax, ./src/mape/mape-syntax. So if you are running mape not from the repository or from the same directory where the mape-syntax folder is, it might not find it. An easy solution would be to copy the mape-syntax directory from the snapshot or from src/mape/mape-syntax to ~/.mape-syntax.

For Map.c scripts that resize the map themselves (for example Goldrush), mape will always take the size specified in Map.c, and ignore its own settings. Otherwise the preview size should work; if not maybe you can show me a script for which it doesn't and I'll take a look.