I bought a hybrid bike from halfords around 6 months ago, however i have only recently had chance to ride it. I have found that the frame is too big, they wont exchange or give me a refund. I was wondering if there was any modifications i could make for it to feel more comfortable?
You have a couple of options, lowering the seat and changing the stem, or some combination of both. One other option would be to find someone that has a similar quality bicycle that is slightly small for them and trade! Local co-ops or bike clubs would be a place to start for this.
As for changing the components you may consider an adjustable stem so that you can do the fine tuning your self, rather than buying a 70mm reach stem and realizing it didn't really help.
You will have to decide what needs to change based on what part of your body is uncomfortable. If you feel stretched out like your reaching to far forward a shorter stem will bring your handlebars in a little.
If you legs are being over extended drop the seat post a little and see how it feels.
One other option would be to show up at your local bike shop with a 12 pack and ask for advice. They will charge for a proper fit, but with beer they should be fine with giving some advice and options free of charge. If they are not, find a new shop.
This site has some good information to explain it to beginners.
As does REI
I ride a bike that is a bit too tall for my daily commuter. Everything else is great, but it is a tall bike for me, so that I have to lean it over a bit when I stop at a light, or tip toe if I keep straight up with my butt on the saddle. I'm fine with it and it is still comfortable for my commuting needs. I don't think I would ride it with clips or straps, though! People ride actual double-frame tallbikes, so I don't see a fatal error here.
If you are unwilling to take the comment advice above, and are committed to riding on this frame and modifying things, you could look into moustache handlebars or cruiser handlebars. They are swept back more than the standard hybrid flat bars, which will give you a hand position that is closer to your body, and should make your ride significantly more comfortable. You'll have to redo your whole cockpit with new cables, and I was a little wobbly until I got used to having my hands behind my steerer rather than in line or in front of it, but it has become natural and enjoyable.
If your frame feels large you can get a shorter stem, lower the seat, move the seat forward, get riser bars or 'trekking bars' rather than drop bars.
You change the position of the bars up and down by moving the stem position on the fork steerer tube and altering the number / position of the spacers.
However, saying that try a bike before you buy is better. And putting miles in will tell you what size you are. All people are different / bikes are different. Otherwise you can go for custom bike fitting.