A GUIDE TO COMFORTABLE SEATING aboard airplanes tells which ones are roomier. Consumer Reports Travel Letter lists among the more comfortable: 747s, DC-10s, L-1011s, A300s, 767s, MD-80s, and DC-9s. The letter says you may feel more like a sardine in the 727s, 737s, and 757s used by many major and regional lines. TOKYO RANKS AS THE SAFEST BIG CITY, according to statistics provided by the Tokyo Metropolitan Police and published by Keio Plaza Inter-Continental Hotel. A survey of seven major cities around the world showed Tokyo had much less major crime in four categories. There was, for example, one murder in Tokyo for every 32 in Los Angeles, the report said. PIEDMONT AIRLINES BIDS FOR WEEKEND TRAVELERS with 40 to 50 percent discounts on first-class travel. Round-trip travel is required, and must take place before noon Saturday or Sunday. And cancellations may involve a penalty. CHINA REPORTS A RISE IN TOURISM. In the first half of this year about 776,000 foreigners - chiefly from Japan, the United States, and Canada - visited China, a 16.6 percent increase over the same period last year. The China National Tourism Administration said growth would continue, because of stabilizing costs of travel and improving services.