I'm an infreqent visitor here but ran across your question. If you are working in GIS, or outputting a series of maps, you're probably used to outputting at regular scales (1:100,000). Guidebook maps tend to be drawn at whatever scale fits the book layout. You also need to make the map large enough to read (especially the busiest area) and small enough that the park agency can put on a cover, agency contacts, rules and regs, park information, and illustrations of newts. If your map will be part of a standard brochure format, printed on standard paper, look up East Bay Regional Parks District (Oakland, CA) and check out their PDF brochures/maps. Good balance of maps and info, in a format that's been tested and evolving for 30 years. It will give you a sense of how the map's lines and symbols work at a standard scale. Also note some small parks are shown big, some big parks shown smaller; most fit on the same basic page.
One step is to find out from your park agency how they intend to reproduce the map. Are they outputting laserprints? Then you'll be using standard paper sizes. If they're having the maps printed, figure out what they usually do, and/or talk to the printer (if they don't have one selected, talk to one or more local printers). You'd have a larger range of paper sizes, but cheapest is probably still the standard paper sizes.
Find out also if the map is being printed in color or black and white/grayscale. If you design the map in color, and find out the budget is for grayscale, there are several ways (in Illustrator, Photoshop, Apple's Preview, and other apps) to convert the map to grayscale; if your colors have good contrast you can make a pretty readable grayscale map even if you didn't start that way). Best to figure all this out fairly early.
If you are printing the map in color, come back here and ask lots more questions (probably a new post since it's a different can of worms).
Also, if the map is being printed, find out if it's in their budget to have the map bleed (extend off the edge of the page). You've seen it before; what it means as far as production is the map art is bigger than the final page, and is printed on a larger sheet of paper and trimmed. It costs a little more and may not really be needed. Since you're doing this pro-bono and they may be on a budget, if you can't get an answer, start out allowing 1/4 to 1/2 inch margin (whatever the laserprinter does or the printer says).
Now back to your question.
When you are sizing to fit a standard page size, scale depends on a lot of factors, and you get to experiment until things work out. Don't worry about making your scale a nice, round number if it simply doesn't work out (map too small to read or falls off the page). Again, make sure the map is readable, especially the busiest area.
As for standard sizes, 8.5 x 11 inches makes a nice tri-fold; 8.5 x 14 inches makes a four-fold map (if the map stays small that means two extra panels for text). 11x17 folds into half, then thirds. Proportionately, 11x17 scales down to about 8.5x13 (legal page minus an inch).
Have fun, dive in, print a bunch of drafts and see what works, repeat.
Hope this helps!