If mounted in a high ceiling building, about 25ft tall, we need to cover 13,500 sqft. sounds a lot but is not that big of an area.
Please, let me know where I can get MR74 area coverage information or how can be calculated.
What type of client are you trying to reach? Most of the time your limitation will be on the client power, rather than the AP itself.
How many clients? Any line of sight obstructions? A lot goes into figuring out your coverage area.
Edit: You might find these helpful.
These are you antenna options:
MA-ANT-20, Meraki Dual-Band Omni Antennas
MA-ANT-21, Meraki 5 GHz Sector Antenna, 13 dBi
MA-ANT-23, Meraki 2.4 GHz Sector Antenna, 11dBi
MA-ANT-27, Meraki Dual-Band Sector Antenna
MA-ANT-25, Meraki Dual-Band Patch Antenna
I would personally only use the dual band antennas. A sector or patch antenna tends to be better when you put in on a wall at one and of the building and point it down towards the other end of the building - often down isles of racks.
Omni's are good when you just want to put them in the middle of everything.
I think you should expect to use multiple access points. Hard to say how many without knowing the layout.
What is the length and width of your space (in meters) and what do you want your minimum connection speed to be?
How can I get the datasheet plots of the MR74 and MA-ANT-27? such as power response plots, frequency plots...