I got some infinity components and speakers that I want to use on my home receiver (Denon does 50rms @4ohm) but I wanted to know if it would be ok to use 5/8" mdf because they are for speakers, no subs in the towers

 

Sure. I just find that 5/8 isn't much cheaper than 3/4 and thinner materials are harder to assemble.

 

how big will they be? midrange carries a ton of energy that cause enclosure resonance and make everything sound muffled and nasaly. Overkill is just enough kill when building speaker cabinets of any kind

 

+1 You can never over-do it. Maybe even look at some sound damping inside the cabinet? Helps seal any gaps as well as add mass/deaden the enclosure.
Years ago I built towers for 5.25" Pioneer components and 10" subs, and they were suprisingly articulate (and a great grad gift!)

 

i was planning on making the towers h=48" w=7" d=7", also i found the infinty rear fills i had also 6.5", i was just wondering that by using 3/4 mdf the cubic feet would still be ok to house 2 speakers and a tweeter