Set the incremental step of HistorySize as 1/10 of current value.

src/EditProfileDialog.cpp

@@ -1071,7 +1071,10 @@ void EditProfileDialog::setupScrollingPage(const Profile::Ptr profile)
     setupRadio( types , scrollBackType ); 
     
     // setup scrollback line count spinner
-    _ui->scrollBackLinesSpinner->setValue( profile->property<int>(Profile::HistorySize) );
+    int historySize = profile->property<int>(Profile::HistorySize);
+    _ui->scrollBackLinesSpinner->setValue( historySize );
+    _ui->scrollBackLinesSpinner->setSingleStep( historySize / 10 );
+
 
     // signals and slots
     connect( _ui->scrollBackLinesSpinner , SIGNAL(valueChanged(int)) , this , 

src/HistorySizeDialog.cpp

@@ -177,6 +177,7 @@ int HistorySizeDialog::lineCount() const
 void HistorySizeDialog::setLineCount(int lines)
 {
     _lineCountBox->setValue(lines);
+    _lineCountBox->setSingleStep( lines / 10 );
 }