4.5 inch line? Is that circumference? Even so, that would be 1.5
inch diameter, still very heavy for a 28 foot boat at the dock. (Or
am I missing something here?)

My primary dock lines are 9/16 inch diameter nylon, and I normally use
4 - two springs, and a bow and stern. (I'm on a face, not in a slip.)
Usually, though, if left for any time, there are 2 additional short
springs that are half inch line. I have a few heavier lines, 5/8
inch, that have been used in a blow, but my home dock is a "hurricane
hole," so I've never used them there.

In general, I prefer thinner line rather than thicker for docklines,
since the stretch reduces strain on fittings. When needed, I double
up, and, of course, proper chafing gear and frequent inspection is
needed. Heavy boats and rough locations need special attention, but I
don't think I've ever seen docklines over 1 inch diameter for a
recreational boat.

By H28 I assume you mean the L. Francis Herreshoff 28 footer. Which
version is yours?

Moores family wrote:
To set the scene-

I've just bought (again, but that's another story) Tropic Bird, a
particularly nice H28 which I have moored at a local marina. I've had
her there for 2 or 3 weeks now, using the mooring lines that came with
her- a couple of 4-1/2" and a couple of 3". I've been going to get a
new set of lines as the old ones are a bit tatty. I thought that 4-1/2"
may have been a bit oversize so I was going to use 3" all round- 2 bow
lines, a stern line and a spring.

Well they're predicting a bit of a blow overnight- 40 knots or so, so I
thought I'd go and check that everything was OK. Everything was, of
course, but when I looked at the 3" spring, which would take the main
force, in the light of a 50 knot blow, my lesson was immediately
learned- I doubled it and made a mental note that peace of mind in a
blow is worth the lines looking oversize when everythings balmy. 4-1/2"
all round it is...
JM