The yeast don't do the extracting, your mash does. Whole corn must first be malted/sprouted before mashing with barley. Flaked corn, or flaked maize is already malted. With sweet feed, you're not really mashing it so much as you are rinsing the molasses off of the grain. Yes, since your goal is to extract as much starch as possible from the malt, to be converted to sugar by the mash (and tannin's are of no concern because you are distilling) then you'd want the grains fine. But you don't want it so fine that you have flower or else you'll have a stuck run off and won't be able to sparge. Crushing the grain is enough. Use some rice hulls in your mash if the grains are ground up too fine. They add nothing as far as nutritional value for the yeast but will ease run off. Typically, you're not going to be adding any yeast to the wort/wash with any grain left in the fermenter. Strain/run off the sweet wort/wash from the grain before pitching the yeast. The sugars are already suspended in the wash and the grains are useless at that point. Leaving them in is more trouble than its worth. Straining the grains off as soon as they are unneeded mean more room in the fermenter for more wash, no additional muddy mess (besides yeast) in the bottom of the fermenter or boil pot to deal with later.
It makes reusing yeast easier, less gunk if it pukes, etc...

Sure thing man...
Here is some more good info: http://www.howtobrew.com/section3/index.html
Since you are asking about corn, I'll assume you are making whiskey. Since whiskey mash is mostly corn... Corn does not have enough enzymes itself to fully convert the starch to sugar so barley malt is added in the mash to aid the starch-to-sugar conversion.
When people here are talking about (any) grain; Corn, barley, rye, etc... Remember that they are always talking about malted grain. Or, grain that has been sprouted so that the starches & sugars are released to provide food for the young plant. That is what gets extracted/converted.