I always advise everyone to do everything possible to reduce the heat-gain heat-loss before you do the manual J calc. Then down-size as much as possible, within limits.
We could cut residential heating and cooling equipment size in America by 30% to 50% if Contractor's would perform accurate Manual J calculations and provided full credit for every load reducing element or detail they can when doing the calculation audit. Additionally, load reduction remedial actions should be provided as options toward further reducing Air Conditioning and heating equipment sizing.
Down sizing has many advantages; the existing duct system & air handler will function more efficiently helping to achieve the unit's nominal tonnage capacity with less than peak heatload conditions. That improves EER & SEER performance.
You might be amazed at what a fully functioning system can cool. I cool perfectly nearly 900sf first floor of an old 1937 farm home with a Half-Ton 6,000-Btu/hr window unit.