You can also use triggers to force a level cap lower than the maximum in the constants for specific units by setting their EXP gain rate to 0. This could be used if you want to limit some heroes to level 30 while allowing others to reach higher levels like 60.
Also you could scale your map so heroes gain more with level ups (so fewer maximum levels are needed) but take longer to gain levels. The standard experience gain rate constants and experiece requirememnt constants are also in Gameplay Constants.
If you are using the default ability learn system that heroes in WC3 use, it can be a good idea to limit the maximum number of levels a hero can gain to approximatly the amount of ability points he needs to spend (or less). This emphisises carefully choosing skills and improves skill efficiency (if you ballence right) rather than maxing all skills early on and then just gaining stats there-after. Can also be done by awarding fewer points per level up (like 1 ability point every 2 levels). If you have few skill levels and a huge level range, consider scripting abilities at certain levels and make them scale with level/stats to keep aiblity usefullness all game.