in my experience, around 5x7 is the size i can tell a difference in prints made from my 35mm and medium format cameras. the difference is not very noticeable in a 4x6, very noticeable in an 8x10. however, just because an enlargement from 35mm does not have as great of sharpness and gradation, and has more pronounced grain, than a print of equal size from a mf neg- this certainly does not mean such a print has any less artistic value. it simply depends on the look you are going for. after a couple of years of shooting exclusively in medium format, i have sold my 6x7, have shelved my 6x4.5, and am using 35mm exclusively now. the look of a 35mm enlargement simply suits my artistic vision more, and sharpness and fine detail are not my biggest priorities, personally. i typically print my 35mm enlargements at 6.5x8- my prints (b&w, usually with high speed film) at this size are grainy but still relatively sharp for 35mm.