I usually bake my bread in loaf pan size 7/3.5 depth 2"and bake for 60 Min. I have to make a large number of loaves so will use pan size 5 3/4 x3 x 2 1/8 so how do I adjust for the cooking time or perhaps I don't need to?
Well, partially this depends on what kind of bread you are talking about. There is likely to be a bit of difference in how you scale the baking time depending on if it is a really wet quick bread (generally made with baking powder or baking soda like most banana bread) versus if it is a yeast leavened bread. Depending on the type you may need to adjust time more or less. Additionally, the timing could be affected by many other factors including how many pans you have in the oven (2 medium pans will bake very differently than 6 small pans if the oven is crowded and there is less heat circulating). Similarly, timing could even be different for different loafs if when you say your are making more loafs you end up baking on multiple racks in the same oven at the same time (top and bottom rack in the oven may not bake at the same rate).
However, the better answer is that the best way to know if bread is done is by using a thermometer. Depending on what sources you read the list slightly different temperatures, but most sources I've ever read agree that most bread tends to be done when it reaches an internal temperature between 195 - 205 F (ex: a probe thermometer with the probe inserted into the middle of the loaf, if you don't want to take every loaf's temperature, at least take the temperature of the loaf in the coolest spot in the oven). Some other cues to look for in knowing if bread is done is for quick breads, make sure the probe thermometer and/or a cake tester inserted into the center come out clean and not sticky with gummy dough (just like checking if a cake is done). For yeasted breads, people often say it should sound be firm on the bottom and sound hollow when tapped on the bottom, but that may or may not occur depending on the type of pan you are using (shiny aluminum pans bake very differently than dark non-stick pans).
Sorry I couldn't give you a simple answer, but there are too many factors that could affect the timing. My recommendation would be to set the time for notably shorter on the first time you bake in the new pans and then check it for doneness and adjust accordingly.