When it comes to the acreage, you need to consider "highest and best use." If you sold your home on 5 acres for $500k, would you be able to sell the 15 other acres for $50k, $100k or $300k? What value is the land? Is it just for beauty? If the home next to yours is $500k and only has 5 acres but they see the same beauty in your 15 acres, why buy the cow if you can get the milk for free?
If the land is used for farming and the expense to farm 20 acres costs more than the profit but 5 acres can be managed for personal use, why spend $100k more if you are not going to get a return on the extra land?
You need to think of your extra land the same as some people consider a swimming pool. If someone wants a swimming pool, they will pay extra for it. But if your buyer has a toddler and worries about he danger and the cost of maintaining the pool for it's limited use is not worth the effort, the pool lowers the value of your home.
You might consider getting the 15 acres appraised and donating it to a charitable organization for tax benefits and selling the home and 5 acres for current market value.