I tried searching this and really only came up with what people normally use on than setups. My question is, does it really matter the size of the mono backing for your braid setup on a baitcaster? I purchased PowerPro 40lb braid and I got a spool of BPS excel mono 20lb to use for backing. Is the 20lb too much or does it not matter? Some say they try to match the diameter of the braid, but that seems like something only that matters for a leader. What are your thoughts here?

No not really I just use some old 12 lb sufix Elite amd it works fine

Smaller diameter line lays better, in my experience. When you use such a heavy diameter, you're also going to be tying a big knot. A big knot can sometimes leave a big lump in your line when you spool over it. I use 10lb backing for everything. I bought a big spool for like 8 bucks at cabelas, has 1725 yards or something. I can back a large number of reels with that, especially since I reuse it the majority of the time.

#10-12 big game is what I back all my reels with. A 600 yard spool is like $8 at Walmart.

Thanks for the tips. Maybe I'll return it then and get a 10lb in the same line. It was pretty cheap either way.

I use the cheapest line that Wal Mart has in usually the smallest they have. I think the last stuff I got was like 6lb Cajun red 300 yards for like $3 on sale.

I would match the diameter.

Match the diameter if you use a minimal amount of braid leaving a slight chance you'll get into the backing. Otherwise It doesn't matter. An excessively heavy backing could result in a splice knot that creates a bulge and disrupts line lay. 

Agreed. Match line diameters. I generally use 6 lb for 10 lb braid, 8 lb for 20 lb braid, 10 for 30.

I match the diameters as well. About 15 yards of line will do for backing.

I have always used 10lb mono backing for everything, regardless of what goes on top of it.  No problems.

Thanks all. I bought the cheapest 10lb mono I could find. 500 Yds for $1.99. Perfect!

What has worked for me, as an alternative to backing braid with mono on spinning reels is to CAREFULLY build up the inner diameter of the spoon using electricians tape or that silicone tape.  I stretch it very tight, and when I do it right it doesn't affect how the line lays on the spool at all.

The braid tends to dig into the tape better than the bare spool and I haven't had any line slip issues.  Just making sure, I put a dab of super glue on the knot where the braid toughes the built up spool.   No issues so far.   Of course. I've never run into a fish that even started to spool me on any of the spinning gear that I own.  Maybe some time, I hope I  have that problem.

You don't want to tie braid to a bare spool, I use blue painters tape and it works just fine, I put tape on when I'm using mono too.  I have been spooled both by fish and boats, best to try and break off your lure before you get spooled.  In the event that you can't break it off, I'd be willing to bet in most cases the knot is not going to hold.  

I use the cheapest line that Wal Mart has in usually the smallest they have. I think the last stuff I got was like 6lb Cajun red 300 yards for like $3 on sale.

I do the same usually Shakespeare 8lb mono for 2.97 800yd spool/

#10-12 big game is what I back all my reels with. A 600 yard spool is like $8 at Walmart.

 

I do too, and then add a topshot of Big Game #10-12, and forego the knot...