Lots vary in size. If the lot is in a subdivision there should be a plat with the dimensions of your lot (Subdivision Plat Book) in the Clerk of Courts office. You might also check the tax assessors office in your parish for a plat but in my parish subdivision plats are in the clerks office.
Tiger,
You say "all of that part of Lot 5". All that part of Lot 5 being what? There is probably more to that description, saying what portion of Lot 5 they are describing. Also, in some cases portions of sections are divided into "lots" (usually irregular sections), by the original government survey. If you can post the entire legal description I can probably help you some. I am a Professional Land Surveyor. (but no, I do not want to survey your land, just to help you).