Some options:
1) If your assessor has GIS, usually you can measure distances and areas with the parcel line overlays. Inexact, but better than nothing.
2) Often, the deed will state the exact acreage and or distances, which are pulled from the actual survey/legal description.
3) Speaking of surveys, if your county allows you to view them, look at the survey directly.
4) I have never had any kickback regarding parcel dimensions and lot size as long as I explain what I did, and efforts to find exact measurements. Even on FHA. If any lender wants better, and its not readily available to review, they can supply the full legal/survey or whatever else is needed. Legals in my area often do note the actual metes and bounds calls. Same with deeds, which just include the legal.
5) There are programs which will calculate exact lot size given the precise dimensions, but you need to input both the angles AND distances, which usually are not available without a full metes and bounds legal description, especially when a stream or road forms part of the boundary. This seems like overkill when an estimate will get you within 1% usually.