I'm going expound on this statement. The only one I know of that put a "number" to it was Hoerner in one of his aerodynamics books. For struts that number was a c/t ratio of 3.7 or t/c of 27%. That is based on fairing thickness NOT tube diameter or member being faired thickness.General rule is the chord of the fairing should be aporoximately 4.25 times the thickness of the tube you want streamlined.
I thouroghly respect Mr. Hoerner and all he has done for aviation. But, some of his 1960's experiments and modelling have been vastly expanded upon by the likes of NASA (as a major example).
It has been found that more elongated shapes are more effective (better laninar flow) - the widest point more in the range of 40-50% from the leading edge (think more like a cigar). And streamlined symetrical airfoils with a length of up to t/c 9% are also more effective (diminishing returns beyond that). 9% probably not very practical and elongated streamlined shapes more difficult to build and heavier, so may not be worth the effort for likely minimal gain. So as a guide, I would say min. 27% to max. 9% t/c, keeping weight to a minimum.
I do agree with Lowell that the fuselage (door) to wing intersection could benefit a lot from streamlining that intersection.