Bigger is always better.... right? You have a critical application that you are really concerned about... so why not just go up a few sizes to make sure the coupling really does its job?
Unfortunately, this logic does not apply well when it comes to couplings, and the reason is three fold.
1. Weight - When you oversize a coupling, you are introducing a greater, unnecessarily heavy, overhung load onto the shafts of the system. This load will lead to a greater bending movement in the shaft... which can impart increased vibrations in to the system (not a good thing), and can also put additional strain/wear on the adjacent bearings and seals.
2. Space - A larger coupling will take up a greater amount of space, generally in both length and width. Assuming system space constraints are an issue, this can make fitting/installing the coupling much more difficult (if not impossible) on the shaft. Creating room for the increase outside diameter of the coupling can also be an issue.
3. Cost (upfront & hidden) - Broadly speaking, most top tier coupling companies take their catalog values and service/safety factor values very seriously... so, assuming you've picked the right coupling type for your application, by closely following their sizing instructions... you will arrive at a coupling selection that is designed and optimized to serve your exact application (without extra space or weight). A larger coupling will cost you more from the manufacturer up front. It will cost you more to ship (as it is heavier). It will likely cost you time (as it is heavier) to install. And, if/when system vibrations and/or bearings and seals start wearing out... it's going to cost even more.
Now, if you just don't trust the coupling manufacturer or supplier you are working with... that's a whole different issue. If that's the case, we recommend you stop using that source immediately. (When you discover the cheap lawn mower from a sketchy brand isn't working well for you... you don't go buy the bigger model from that same manufacturer. Right? You find another, more trusted brand/company and go with them instead.)
And, if you are working with a brand you trust... but are concerned that you may not have sized a coupling correctly... and just want to talk to someone before committing to a specific design... pick up the phone! (In our humble opinion, when you dial the right number, you'll most certainly get a friendly face in Downers Grove, Illinois on the other end ready to help.)
Recommended Follow-On Reading: To help you in selecting the right size coupling, find out how to quickly calculate coupling sizing torque here: Coupling Sizing Torque - How to Quickly Calculate (inclusive of Service Factor)