Ye Olde Inn

Does anyone know the figure size for the original US Hero Quest. I thought 25mm and bought some 25mm figures, but they are much smaller than Hero Quest figures. Thanks.

Would be "heroic 28mm scale" so around 29mm - 30mm for an average human height. You can probably get away with using anything labeled:
28mm scale
Heroic 28m scale
30mm scale

Hope that helps!

Original HQ minis actually had a number of slightly different scales in practice.

Technically figure scale is based on the tip of the toe to the centre of they eye of an average (5'8") man.

Thantos has the right idea though.

I third that, though most 30mm figures will already be too big. The Barbarian is my direct comparison piece here.
If height and proportions are supposed to fit, you can proxy almost only with Games Workshop figures from the '90s and noughties.
Other figures fitting in style and scale range from Otherworld to Excalibur to a few obscure manufacturers.
The most useful, though, are those from other older boardgames like DarkWorld and Legend of Zagor.
Sadly, these also cover a lot of the same monsters HQ has to offer...

How tall is a real orc?

For anything other than humans and real life animals that you want to match in scale, you can use pretty much any model that fits on the board, although anything close to 28mm would work best for monsters that you want to roughly match the common ideas of creature sizes..

That seems about right. Thanks.