Prop Size

I have an 88 270 Dancer and need to replace the props. She has twin 4.3L V-6 205hp engines. I know the chart says 13.75 x 21, however I was looking to see if anyone had any other recommendations. Someone said I may want to look to switch to 14.5 x 19 to better work the engines. I don't know much about engines or props, so I was hoping I could find some help here.

I'm no expert...but have bought a few props in the past. You should stay with the Mercruiser spec's. I have bought stainless steel turbo props, and you may get a couple more Mph's.. due to the fact they are cupped and thinner...but they will cost ya about 1,000 as opposed to 1/2 that for a stock prop.

With full fuel and full water, you should be at max WOT or max WOT plus 100 or so RPM's. That way when you start loading the boat with provisions, the RPM's will drop into mid to top of the range.

Look into Piranha Propellers. I used one of their 4 blades with tremendous results. I picked up hole shot, top end and MPG while reducing GPH by a factor of 2 across the RPM range. They used to have a blade exchange program while you are dialing it in. Worked for me.

HTH.

You don't say why you need to replace your props. If I assume they are damaged beyond repair, that also means we can't tell much from them about your current performance.
I would stick with 3-blade props because you have twins and v6 engines. Stainless is better if you can afford it. That will provide better performance and should hold up better.
If you can't compare to current performance because your old props are damaged, you'll have to go with the factory props, or you can go with 19in if you want. Some dealers participate in Mercury's trial program for props where you can try different props to see what works best. Of course, it's best to have everything working right before this - tuneup, clean bottom, clean outdrives, desired loading.