Bicycle Mechanics - Rider sizing

Does anyone have any suggestions on bicycle sizing? I currently have a 58cm frame that seems to fit me rather well with one exception. My arms seem to get very tired early on in a ride. I feel as if I have to hold myself up more than I should. Am not sure of the exact length of my bar stem (came on the bike, has to be at least 100mm or better though), however, I am thinking it may be too long.

Am not an expert on bicycle sizing so I do not know if this is the issue.

Any help would be appreciated.

You can easily measure the stem, from centre to centre. Also, measure your top tube length, centre to centre, since the 2 combine to give your reach.
If you play around with shorter stems and find one which is comfortable, you will be able to measure and replicate that position on other bikes. There is no useful formula or guide as to what is the "correct" reach, it depends on your flexibility, build and experience, and will probably change over time.

It is hard to measure angles on a bike,but they contribute towards things like the layback of the saddle. I like to measure the bike on a grid , in [x,y] with the origin [0,0] at the centre of the bottom bracket. Use a plumb-line to mark the BB position on the top tube, and measure back to the saddle nose, and forward to the bars, from this position. You can then be assured of duplicating the 3 points of contact (pedals, saddle, bars) whenever you need to.

It may not be size at all. If your saddle is tilted down
in the front it will pitch your weight forward and
put alot of pressure on your arms (which support
all your weight). Also check saddle to bar drop which
may be too much in your case, if its this either
lower your seat a small amount or raise your stem
(if its threadless may have to get new stem with
a bit of rise). LBS should be able to assist here.

Marty

Rivedell Bikes (http://www.rivbike.com/html/rr_comfposition.html) probably have the simplest advice on a achieving a comfortable position. Not designed for racers or serious off-raoders, just your average Joe on the road.

The traditional first-order approximation for setting handlebar reach is to place your elbow against the nose of the saddle and to select a stem which permits your fingertips to touch the straight upper part of the drop bar. Of course, this is tightly interrelated with the saddle's fore-aft positioning, which can also affect the load on your arms.