“I have a Sony Handycam DCR-DVD105 (about 3 years old). I have an iMac which loads CD on the side (of the screen). What do I need to buy so that this smaller CD can be inserted?”
I’m not aware of an adapter I would feel safe recommending for inserting mini DVD media into any slot loading DVD drive like most iMac and MacBook hardware now uses. Rather than risk breaking your internal drive, you might be better off investing in an external USB drive to playback the mini DVDs from your camcorder.
In my own experience buying external CD and DVD drives, there are several that work reliably with Mac hardware. I own two different units from LG that are Mac compatible, as well as a Sony drive that I believe I’ve connected to a Mac in the past. Most external drives are priced at $50 or less and should solve your problem without putting your Mac at risk of eating the mini DVD and requiring additional repairs.