Usually at the helm console of any boat of any size, the boat compass is mounted directly behind the helm wheel. If there is a compass on your boat, then mount the chart plotter several inches away. I don't think a typical chart plotter will have any sort of permanent magnet inside that can distort the compass, but in some models a magnet is used to retain certain opening doors. You certainly don't want any magnets near the compass. Check the chart plotter for use of any magnets as door latches.
If there is no compass, you are free to mount the chart plotter where you like.
As for what model and size, this is usually determined by the budget. Many years ago I spent $1,500 and got an 8-inch display on a top-tier product (a Lowrance HDS-8 first generation). I am still using that device after what seems like an eternity and the introduction of three newer generations in that product line.
In buying a chart plotter with fish finder, I would look at the following aspects:
- does it have NMEA-0183 ports? I'd want at least one and preferrable two.
- does it have a NMEA-2000 port? This is a must-have feature.
- what navigation systems can its GNSS receiver use? In addition to GPS the receiver should be able to receive and make use of GLONASS and GALILEO, and use augmentation from WAAS for GPS.
- what electronic chart cartography can be used? There must be suitable charts available for the waters where the boat will be used; in my case that means good Canadian chart coverage.
- what SONAR technology is available? I expand on this below
In establishing a budget, allot at least $200 for additional electronic charts.
In regard to NMEA-0183 ports, you will want to interface the chart plotter to your VHF Marine Band CLASS-D DSC radio--I assume you have one. Many radios need NMEA-0183 inputs for getting automatic position update data to them. There is a trend in lower-tier chart plotter products to omit a NMEA-0183 port. Be careful if buying a chart plotter without a NMEA-0183 port that you don't create a mess for interfacing to the radio.
SONAR technology is now very complicated. Traditionally a monotone pulsed SONAR was the norm. Now we have enhancements like frequency-modulated continuous-wave SONAR (often referred to by the term "chirp" or by a fanciful and inaccurate acronym CHIRP), side-scan SONAR, forward-looking SONAR. You have to decide how much SONAR you need. Do you want to just find the depth to the bottom? Do you want to have frequency agility to tune the SONAR frequency for certain species? Do you want to have superior target resolution in water depths greater than 200-feet? The field of SONAR devices available now spans an enormous range of technology and price. Without knowing your requirements, no recommendation can be made for a particular device.
As for brands, there are several major families:
--NAVICO, with their Lowrance, Simrad, and B&G lines, all with many models
--GARMIN, with many, many models
--RAYMARINE, with many models
--HUMMINBIRD, particularly popular with freshwater anglers
--FURUNO, particularly popular with professional saltwater boaters and anglers
I can't make much sense of all these choices myself for my own use, so I cannot really offer any sort of well-thought-out advice to you on a particular brand and model. It will always come down to your budget. You should mention that to begin to limit options.
One comment I can make: unless you want to upgrade the chart plotter with fish finder frequently, buy one that you can be happy with for the next five to ten years and figure its cost will be amortized over that period. Look for good support and longevity of support for a product. Also consider buying a used device from someone who loves to buy the latest and greatest every 18-months.