I have a tendency not to prune/rejuvenate shrubs unless they need it. Forsythia flowers on stems and buds made in the previous year so if you prune forsythia too late in the year, you will be sacrificing next spring's flower buds and flowering stems.

Since your forsythia is new - even though good sized - why not let it go for a few seasons? At some point down the road you can trim it up or do a rejuvenation pruning (cutting a third of the oldest stems down to the ground). They respond well to this kind of treatment and really bounce back. But in the meantime, forsythia is prettiest when the branches arch - pruned and sheared to a fare thee well not so much.

That instruction is utterly bizarre. Why would anyone cut a spring blooming shrub to the ground in early spring? Very few shrubs get cut to the ground: Buddleja because it is a mess if left, red twig dogwoods to get fresh brightly coloured stems, but not many others. Forsythia doesn't actually need any pruning if it has sufficient space. At least not for quite a few years. If you must cut it back do so immediately after it has flowered. It flowers mostly on old wood so it needs time for that to grow ready for the following year.

After flowering I cut out occasional branches as low to the ground as I can reach them. They do bloom on last year's wood, and look best when allowed to be the fountain shape that is natural. Nothing annoys me more than trimming them into buns or squared off hedges, and that also, usually, means a weak bloom the next Spring.

Yes, nothing sadder than a forsythia tortured and sheared into a perfect monolith with a few random blooms.

Don't have forsythia myself, but my untouched Dart's Gold Ninebarks are things of beauty with their long, graceful arching branches.

as a general rule.. you prune flowering shrubs.. AFTER THEY FLOWER ...

and since this flowers in spring.. INSURE.. you do not prune until AFTER it blooms ...

as to why to prune... its all about your goal for the plant ...

google forsythia... even better if you have the cultivar name ... and go to the IMAGES .. and just look at how various plants are maintained ...

and then prune yours to accomplish.. whatever 'look' you like ...

post a pic if you want ideas specific to your plant...

ken

Most people don't give it enough from to 'do it's thing'. People won't even cut out some lawn to give it room so it doesn't have to be pruned. They move well when dormant so give it plenty of room.
I too, like the natural unpruned look of Forsythia.
Mike

Thanks everyone for the input. I won't be pruning this (unless it needs it at one point) that's why I bought/fell in love with forsythia, the cascading limbs. . . it has plenty of room to reach, spread and sprawl all it wants. . . thanks again. . .