FLORA Cheong-Leen looked as if she had been poured into her white bell-bottoms. But not the matching jacket. ''Size 36,'' said the ex-dancer, ex-model, designer. ''Bigger jackets sit so much better.'' Has the trim, terrific body which once graced Britain's Royal Ballet been expanding lately? Not so you'd notice without the aid of a magnifying glass. On the other hand, her clientele in China has been growing. And like Flora, the mainlanders just love that roomy feeling, if for somewhat different reasons. ''They figure that because more fabric and labour have gone into a garment, they're getting better value for money. Then there's the modesty thing - loose clothes are less revealing. What they don't like is pure, natural fabrics like silk or linen which wrinkle easily and are hard to clean. ''The first thing Chinese customers do is crush the material to see if it keeps its shape. That's why they prefer blended fabrics and synthetics.'' Is it possible that behind the thrice-married social butterfly lurks an astute businesswoman who has done her market research? Why not; she's been in the rag-trade for more than a decade. And if you want proof, just look at Flora Cheong-Leen's current Summer 93 collection for her Pavlova shops. They are aeons removed from the current vogue for pared-down classics in neutral colours. Agreed, Cheong-Leen has her share of flowing numbers in black, white, beige et al. But you will grow old before you find a Pavlova outfit that is unadorned, and forevery neutral there are loads of colours. Flares with bows at the knee, blouses loaded with ruffles, suits edged in bold braid, eveningwear that spells frou-frou from neck to hem, combinations like yellow and kingfisher blue. Frankly, no one over 25 should be seen dead in those clothes. That's just the point. And the secret of Cheong-Leen's success. Call her clothes kitsch if you like, but they have real appeal to a sizeable section of the market: all those fun-loving young things who love their fashion feminine with a touch of fantasy. With competitive prices like $2,500 for a suit, $1,800 for a jacket and $900 for pants (reduced by 20-30 per cent during the current summer sale), quality fabrics including pure linen, a custom-tailoring service and more sophisticated lines for the young executive woman, Pavlova occupies a firm position among local designer labels. There is more than hint of Cheong-Leen's ballet background in her designs. From her jaunty bellboy suits to her mini party shifts with their layers of chiffon, there is a strong feeling of theatricality that clearly has its origins in her London years. ''I studied costume design while I was at the Royal Ballet,'' she reminds. ''I made my own tutus, helped with make-up for productions like Swan Lake and learned everything I could about wardrobe.'' It comes as no surprise that Pavlova with all the trimmings is doing well in with fans in Shenzhen and Guangzhou - and soon the ones who are waiting for her new shop to open. ''That will be in Shanghai next month - my third shop in China. I'll also be going to Beijing and Dalian in early September for a big model competition and a fashion show being co-sponsored by the Trade Development Council. ''It's going to be really avant-garde and several top Hongkong designers including William Tang and Walter Ma will be taking part.'' With eight boutiques in Hongkong, plus outlets in the Philippines, Singapore and Hawaii, the daughter of former Urban Council chairman Hilton Cheong-Leen has definitely established herself - and the critics who call her a frivolous show pony be blowed.