Use size_t as len type for Meter_UpdateValues

Most of the time the parameter is passed to snprintf type functions

DateMeter.c

@@ -19,7 +19,7 @@ static const int DateMeter_attributes[] = {
    DATE
 };
 
-static void DateMeter_updateValues(Meter* this, char* buffer, int size) {
+static void DateMeter_updateValues(Meter* this, char* buffer, size_t size) {
    time_t t = time(NULL);
    struct tm result;
    struct tm* lt = localtime_r(&t, &result);