My eclipse version is Mars Release (4.5.0).Preferences->XML－>XML Files->Editor,I can not set the value of Indentation size. The "+" and "-" can not be clicked . I can not input value yet!
Had the same problem, I found the following workaround:
- open with a text editor this file in your workspace folder: .metadata/.plugins/org.eclipse.core.runtime/.settings/org.eclipse.wst.xml.core.prefs
- change or add the following line ("4" is just an example): indentationSize=4
- (Re)start Eclipse
Rob van der Most
This is a known bug in Eclipse. It is related to GTK3: https://bugs.eclipse.org/bugs/show_bug.cgi?id=428246
It was originally reported in early 2014 for the Kepler release.
In addition to Kollgandren's answer, if you want to use spaces as your indent, include two options in your ${WORKSPACE}/.metadata/.plugins/org.eclipse.core.runtime/.settings/org.eclipse.wst.xml.core.prefs file.
indentationChar=space
indentationSize=4
来源：https://stackoverflow.com/questions/31747059/eclipse-xml-editor-indent-using-spaces