The best size, or a pretty standard size today, without doing a larger, built-in style is 36 wide 72 tall. That has become pretty much the industry standard in terms of an opening. Depth will be determined by the cabinet space and location, with 24″ being the shallowest considered a counter depth.

yes that will 100% cause erratic temperatures. You need to flush the system out periodically to take care of this. Not an issue with the thermostat.

You can look at this model from LG LFXS26973S, it’s a good value smart appliance. Keep in mind that a smart fridge doesn’t offer a ton of features. Other than remote monitoring for temp and service diagnostics.