hey claire, i would recommend the following:
Singapore Art Museum Glass Hall - really nice and small, about 12 - 15 tables i think. Two cobbled outdoor courtyards and a small room on the side for the bride to change outfits. I've been there for art gallery openings and it looks really lovely with tall candle stands lighting up the area.
Asian Civilisations Museum River Room - also a small area, max about 17 tables, lovely black and white lamps. Outdoor pavilion for buffet spread with tentage... lovely view over the river.
Alexandra Hortpark - my mom loves this area and keeps pushing me to have it for my wedding lunch but i'm not even making plans for my wedding yet

really beautiful location though maybe better for a lunch, so that you can take nice photos with the gorgeous flowers and grass areas.
Raffles Hotel - there's a small grass terrace there. Not sure what it's called and it's quite small but it's really beautiful and secluded.
Sentosa beach pavilions - great for beach dinner, would be really nice with flowers and candles etc
Another thing you could do is book the beach area of Cafe del Mar, with the outdoor pool...might be a bit pricey but you could have an amazing beach wedding dinner here.
If I think of more ideas I'll let you know
