A couple of factors. In an already established weed field their is a huge seed bed and plowing often encourages weeds by activating those seeds. Check with your local extension agent to see if a drill seeder is available to rent. If so I would spray roundup in the fall and then seed the prescribed number of days later with the no-till drill seeder. Depending on what your area is like your mileage may vary on the deer's reaction. I've found they use the food plots during the summer but move off them the minute acorns drop and then don't come back to them until late season when everything else is gone. In that respect they act more like a sustainer for the herd rather than a hunting area/attractant. Another thing you could do, instead of roundup is to do a summer burn of the field, if you can do so safely. This kills off adult weeds but doesn't disturb the seed bed to encourage new weeds and releases nitrogen back into the soil, then drill plant.

As far as size, I've seen that narrow winding plots are more effective than large acerage. It has to be large enough to sustain heavy browsing but narrow enough to make the deer feel safe. A windy road-sized plot with lots of edge is better than a huge field, especially if you plan on hunting over it. If it's a large field they could graze out beyond your effective bow range but if it's narrow they will always be near an edge where you can set up.