Logback roll only on file size

You want a SizeBasedTriggeringPolicy: http://logback.qos.ch/manual/appenders.html#SizeBasedTriggeringPolicy, probably combined with a FixedWindowRollingPolicy.

Was a comment before, not sure it deserves it's own answer, but here it is. ;)

Just to expand on sheltem's answer:

From my experiments, you have to use both a FixedWindowRollingPolicy and a SizedBasedTriggeringPolicy, as documented here. It appears that you can't use a SizedBasedTriggeringPolicy with a SizeAndTimeBasedRollingPolicy, for example: in my case logging then failed to happen.

Typical example in your xml file might be:

<appender name="FILE"
    class="ch.qos.logback.core.rolling.RollingFileAppender">

    <file>../logs/MyProject/testrolling.html</file>

    <rollingPolicy class="ch.qos.logback.core.rolling.FixedWindowRollingPolicy">
        <fileNamePattern>../logs/MyProject/backup%i.htm
        </fileNamePattern>
        <minIndex>1</minIndex>
        <maxIndex>10</maxIndex>
    </rollingPolicy>
    <triggeringPolicy
        class="ch.qos.logback.core.rolling.SizeBasedTriggeringPolicy">
        <maxFileSize>5MB</maxFileSize>
    </triggeringPolicy>

    <encoder class="...

Your backup files will then be called: backup1.htm, backup2.htm, etc.

It would appear that you cannot include a "%d" formatting directive in the fileNamePattern: I tried and logging failed to happen. However, according to the documentation you must include the "%i" formatting directive, which makes sense.

much later

This may be me being naive, but I'd have thought a logging app should be designed to produce output by default, i.e. always interpret directives so that something at least is produced, rather than nothing.