I've got my 2 big trucks insured under another trucks full coverage policy. It's a limitted use policy which more or less means they're not my primary means of transportation. They're also covered for a certain dollar amount, liability and no/uninsured motorist. These rider policies cost me less than $20.00 per every six months. I think your agent is being lazy or just unwilling to help. I've never heard of a size restriction.
Hope this helps,
Scott
