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38/16=2.375 That would be very tall. The engine probably wouldn't be powerful enough for 5th gear to be usable. The taller you make it the fewer RPM the engine turns to go the same speed in the same gear. It also requires more power from the engine. If you've got an 18 or 21 speed bike think of this as using the biggest front sprocket and the smallest rear sprocket. you will notice it's very hard to pedal especially from a stop.
The reason to go to a 16t front sprocket is that it spreads out the gears by making the final drive ratio slightly taller. By "spreads out the gears" I mean you don't have to shift up thru the gears quite as fast spending more time in each gear. It is many people's opinion (myself included) that the stock gearing is too close. The negative side effect of going to a taller sprocket ratio is that it lowers your top speed. About 5-10mph for the 16t sprocket.
With the factory sprocket ratio of 41/15 which is 2.73 in 5th gear you will usually run out of engine power just before or near redline (8000 RPM) which is 80mph.
The 16t sprocket would yield 2.56. So going with a taller sprocket ratio (ie. less than 2.56) you would run into top speed problems due to lack of engine power and going with a shorter sprocket ratio (ie. greater than 2.73) would put the gears even closer together and your top speed would be limited by over revving the engine.
To calculate the sprocket ratio, you simply divide the rear sprocket by the front sprocket to get the ratio.
41/15= 2.73
41/16= 2.56
42/16= 2.63
43/16= 2.69
Hope this explains things a little more clearly.
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