Certainly, in your Dashboard go to Wireless > Configure > Radio Settings. You can select each individual access point and manually set its transmit power in dBm in the right hand menu that comes up when you select the AP you want to adjust. Lowering that value will effectively shrink your "coverage circle". Also on that same page, make sure to enable "automatic power reduction" which might not be on by default, because you obviously do NOT want your APs transmitting at 100% transmit power.
The other thing to consider is on the Wireless > Configure > Access Control page, where you set the minimum bit rate, which also has that "cell sizing" effect, by disabling the lowest couple of data rates for each band. A common setting is 12 or 18Mbps, but it depends on your specific environment and client devices. Note this can be a per-SSID setting.
On your 2nd question, to better know how you're coverage looks, anyone will say doing a "site survey" is the only true way, and that's correct. That can also involve (sometimes) expensive tools and/or professional services, depending on your deployment. But for a simple and easy estimate, just use the built-in local status page on Meraki APs. Associate a device to the AP and bring up a web browser and go to ap.meraki.com and that will bring up a local status page for that AP you're connected to. It will show you signal strength, channel utilization, you can run a speed test... you can walk around with it and basically do you own mini site survey of sorts.
Hope that helps!