Big fish are relative and thats the way fishfinder companies treat them,  some consider a 18 inch bass a big fish others that may be 24 inches....and that just covers the little fishy symbols,  then we throw in saltwater where they may be 5 to 10 times bigger,,,,wouldnt it be nice if we could "set" the big fish symbols and alarms to a size we consider big.

Next you go into arches,,,, a perfect arch is a fish that passes directly thru the middle of your transducers cone,,,and of course you transducer needs to mounted correctly to acheive this.  Then some fish produce better returns because the have big scales over fish that have skin, but what about the fish on the fringes of your cone that have reduced returns.

I have watched in awe at the dramatic improvements of fish finders in the past 10 years and cant wait to see what all happens in the next.  I have a degree in electronics and a background with radar and communications, so this isnt all smoke and mirrors to me, but I'm guessing you want to know what to look for in finding a big ole cat.  Doc has a reply on here somewhere where is uses the double bounce method to make those bottom huggin fish show up---its a real interesting concept and does work...then you'll read about Phil King as he finds suspended blues in his waters and how he drifts bait thru them.  It is sometimes frustrating as heck to figgure out what your looking at especially when the fish ain't cooperating which sometimes seems like most of the time.  For me I like to target the lone fish glued to the bottom and may pass over him several times trying to get a fix on him before fishing for him.

What IMO you want to look for in a fishfinder is the most wattage (output pwr) and largest screen you can afford.  The power is just like illuminating something with a flashlight vs a spotlight. Next is your cone angle---60 degrees generally works best for catfishermen and it gives you a one to one perspective  looking down 50 ft the area on your screen represents a cone of 50 ft across,,now if you have a screen that is 5 inches across each inch of that screen will represent 10ft, so a super trophy cat that may be 60 inches (5ft) will have an arch of just 1/2 inch---if it happens to be in the middle of your screen.....and on the bottom....a suspended 60 inch fish at 25 ft will have an arch of 1 inch.

Are we having fun yet? time for two aspirins or a bottle of jack if ya want to go further  :lol:

Yeah, thats what I wanted so depending on your unit and depth and screen size and type of fish their are to many varibles I'll just have to figure out my own unit on my own and get comfortable with the size fish Im chasing.

thanks , ill try that fish trick i didnt even think about that .

I still dont have a clue how to really get the most out of my finder.I pretty much stay on the same screen all the time. :oops:

I "road hunt" for fish every trip.lol Thats what my wife calls it.  She says that its cheating. Riding around till I mark large fish then catching them.  I rarely ever blind fish. If I don't mark a large fish in a hole I don't fish it unless there's a lot of structure and the bait is deep.  
I have found that any large (over 30lb) fish will have a "gray center" in the arch on both my Lowrance's.  I run one FF with the sensitivity around 60 so I only see large fish and the other on 70-80 which eliminates clutter and I can see bait and stacked fish. If I mark a large fish and they are in the proper location relative to structure or humps (in a feeding position) 80% of the time I catch them.

Ya gotta remember, too, that the arch you're seeing (when you mark a fish) could be ANYWHERE in area covered by the cone of your transducer.  If you mark a fish "on the bottom" in 50 feet of water, it just may not be right under your boat.  It could be suspended off to the side of the boat a certain distance.

I'm not even going to bother to deal with the math...

NOTED:::