@300 dpi an 8x10 inch print is (8x300=) 2400 x (10x300=) 3000 pixels = 7200000 = 7.2MP. @300 dpi a 4x6 inch print is (4x300=) 1200 x (6x300=) 1800 pixels = 2160000 = 2.16MP. Easy math. As stated, you can upsample to meet the 8x10 requirement unless the magazine specificly prohibits it.
Thanks amazingthailand, it's easy math for me now. My plan was to travel to Bangkok sometime during the next year. Will lack of speaking Thai be a problem (I'm studying with a DVD but a slow language learner)?