Joseph, the letter is printed in Igeres Sofrim #29. R Akiva Eger was asked his opinion about tefila betzibur during a cholera epidemic. I don’t know where Avi K got “shorted tefilot” from; if he meant “shorter” then on the contrary the letter recommends *longer* tefilos, because he suggests some *additions* to the regular tefilos. But he says minyanim (in that shul) should be limited to 15 men at a time, davening in shifts, each man should be assigned to a minyan and told what time to come, and the government should be asked to station a policeman outside the shul to enforce the 15-man cap. If the government is reluctant to send a policeman they should say that R Akiva requested it, and he is sure that his name will do the trick.
He also gives other advice, including going out for walks in the fresh air, keeping rooms ventilated, etc.
I *assume* (but this is only an assumption) that the 15-man limit was based on his personal knowledge of the size of that shul, and that in a bigger venue he might have prescribed a higher cap. My own opinion about the current situation is that shuls should remain open with strict caps on capacity (I would hope there is no need for a policeman), people should sit apart from each other and not walk about, touch points should be disinfected several times a day, and of course anyone who is particularly vulnerable should not be going *anywhere* including shul.