Try setting DefaultFont to "8.Helv". IIRC this is the default on DBCS versions of OS/2 and so most of the system dialogs should scale fairly well with it.
IIRC the IBM help viewer uses hard-coded fonts in some places, in which case there's nothing you can do about that except maybe use NewView instead (which I would generally recommend anyway, due to clipboard support, etc.).
BTW, if you see clipped dialog text with DefaultFont, it's a bug in that application, not a bug in OS/2. All dialogs are supposed to be designed to fit the default dialog font (normally System Proportional), whether or not they actually use a different font for display. This is not unfortunately very well documented for programmers. However, the fact that the DefaultFont setting causes system-wide rescaling of dialogs is not only the correct behaviour, it is important that it work like this as some environments (including all DBCS versions of OS/2) depend on it.
If you don't have such an environment, then the patch that Andreas mentioned may well work for your purposes... but you do need to understand that this patch works by deliberately breaking part of the correct Presentation Manager behaviour. It's a trade-off, and you should understand the ramifications.