Inside size is 14 1/2" x 14 1/2", I have used a rectangular baking dish and flipped the rotating tray. They tell you not to, but I have had no problems and rotate food 1/2 way through cooking.
cavity size is 15 21/32" wide, 10 7/16"high, 15 27/32" deep, unit also has a glass turntable, approx. 14" in diameter. The back of the cavity is curved, no square corners. A baking dish would have to fit on the turntable.
Not sure what a trim kit is, but nothing like that came with this microwave. There was also nothing you could get as an accessory. So I would say no there is no trim kit.
Hi Ron, Thank you for your question on the LG 1.4 cu.ft. Mid-Size Microwave Model # LCS1413SB. There isn't an LG Trim Kit available for this unit, but please contact Customer Information Center at (800) 243-0000 to verify this...IFV
Thank you for your inquiry. The owner's manual for the LG 1.4 cubic foot Mid-Size Microwave, Model # LCS1413SB does not have any instructions that show that muting the beep is possible with this model...^IFV