You can achieve that by going in the "Properties" pane of your text box and in the "Display" section you should modify the content of the "Text" field as following: between curly brackets type the number of characters you want to be the maximum (e.g. {7}).
Can you please send us your AIP (project file) to support at advancedinstaller dot com so we can investigate its settings? Also, please specify which control is the one you configured like so.
We have tested with both MSI and EXE builds and could not reproduce your issue. Could you please send us the .AIP (project) of the installation to support at advancedinstaller dot com so we can investigate it? If not please send us a test .AIP project that reproduces this issue. Thank you!
We have investigated your project and, indeed this is an Advanced Installer Bug affecting only "Surface Theme". A fix will be available in the a future version of Advanced Installer, thank you for bringing it to our attention. Until then you could use any other theme. Please note that when changing the project theme all the custom changes you performed for the current theme will be lost. The new theme will use in place its default dialogs.