You can't just decide AVR according to a room size, however you have mentioned in your post that your listening distance will 7 feet so both Denon 1612 & Marantz SR5005 are powerful enough for your room.
Following Points one has to keep in mind while buying AVR
Sound Quality :
Speaker Calibration
Power
Wireless Connections : Bluetooth, Ehternet, Wifi
Outputs : No.of HDMI out etc
Inputs : No. of Optical , HDMI
Apple Connectivity : iPod,Iphone
Video Up conversion :
MultiZone
No. Of Channels : 5.1 and 7.1
Sound Decoding : DTS, DTS HD , Dolby etc
Sound Quality should be the most important while choosing AVR and then make sure that the amplifier can deliver clean power to the speakers. Compare power ratings between receivers The smaller the THD rating of a
receiver, the better quality sound it produces.