Even with ideal growing conditions, Helleborus increases in size very slowly. I have 25 year old plants that are no more than 2 feet in diameter.
I have concluded that; A large measure of patience is necessary!

Soil pH is important. Circumneutral (a pH of about 7.0 ) or slightly alkaline soil would be best, but they can also tolerate soils that are somewhat acidic.
In my experience, they perform poorly in very acidic soils and will remain about the same size, year, after year.

If you are uncertain about the acidity and fertility of the soil where you have the Helleborus planted, contact your local County Extension Agent for advice about collecting and submitting soil samples for lab testing. You are usually able to specify the type of plant material you want to grow and receive a detailed report for any necessary chemical/organic adjustments.

Most of the Helleborus x hybridus plants produce viable seed (I have a few that never have) and over the years, if the seedlings are left in place, large colonies of plants will form, covering many square feet and most often, with a variety of colors.

Like most plants, hellebores are tolerant of a pretty wide range of soil pH so I'm not sure I would be all that concerned about adjusting to a specific level. Most wholesale growers and breeders consider anything from 5.5 to 7.0 appropriate and I've never found my naturally acidic soil (6.3) to be a hinderance to their development.

But they do take some time to develop into robust clumps. And it does help if the soil is quite fertile and enriched with quantities of organic matter. After than, it is only a matter of patience :-) My older clumps (5+ years) of xhybridus are anywhere from 2-3' across and produce dozens of flowers each season. H. argutifolius grows faster and by nature is a larger plant - 2 y.o. plants are easily 3'x3'.