I don't know whether this is the case for you, but in general I recommend not to use large design sizes for posters.
As you can see from the few fonts which actually exist in 17pt design size, this was meant for a few words on a title page, if at all. I find
cmr17 immensely hard to read because the strokes are so thin compared to the size of the letters.
If you look in the TeXbook, you'll see that Don Knuth himself didn't use
cmr17 at all (which probably wasn't around at the time of writing it), and
cmr12 only once, for a concert program. He used
cminch for the title page.
The larger the design size is, the harder the font gets to read, in particular for cmr. cmr5...cmr17 are all meant for a reading situation when you are holding a book in your hand. Standing in front of a poster is completely different. Larger distance, inconsistent lighting, bad angles. You move while discussing different things, people are moving around you. You should do all you can to make it easy to read.
I advise to make the poster as a normal document at much smaller page size (with fonts of usual design size) and magnify when printing.
To illustrate the effect on readibility, I made the following example:
\font\testv=cmr5 at 10pt
\font\testviii=cmr8 at 10pt
\font\testx=cmr10 at 10pt
\font\testxii=cmr12 at 10pt
\font\testxvii=cmr17 at 10pt
Decide for yourself which is more readable ;-) Btw, acroread is doing a great job on enhancing readibility on screen. Printed the effect will be much more prominent.
As I said, it may well be that all this isn't relevant to you.
But for posters authors are frequently searching for a "14pt" or a "17pt" option in the hope of getting larger print, and this is wrong. Even "12pt" is harder to read for cmr in my opinion. If this is not the case for you, fine. But a title in cmr17 should be really short to be still readable, no need for maths there.
So just sayin'