Forsythia is very easy 3mm cuttings would be ok. No need for rooting hormone. Just stick those in soil and outside and within 2months there rooted.
I do scar my cuttings at the bottom. That means scrape little bark away so they have wounds. On those spots i put rooting hormone. on the edges of the wounds will be roots growing. (on most of my cuttings)
I think it is important to realize that -besides a few easy to root plants such as juniper, willow, elm, forsythia) many tree species will have a preferred method and time of year for rooting. You will really need to look up information on how to root individual species, which age the cutting needs to be etc. If you know what to do, many species become easy to root.