Andy: I would say you are between sizes but a 53 would be my first guess (my daughter is 5'6" and easily fits the next size down, the 48). The difference in top tube is 1.2 cm. I have to agree with fatboy that there are quite a few options before going to the expense and trouble of changing bikes. I'm not even sure that the smaller bike will solve your problems. First and foremost fit a shorter stem, possibly with a steeper angle to bring it higher. Then persevere by doing lots of frequent shorter trips (say 5 - 10 miles at the most). Do lots of stretching and circling of your hips and shoulders. Adjusting to a new bike is more significant than people often realise.
PS I wrote the last couple of sentences before I realised you already had a road bike. If your road bike fitted you comfortably and you are used to a road bike riding position then it should be easy to exactly measure the critical distances on your road bike and set up your Horizon to the same measurements. Generally a road bike will be the tougher riding position. If you are used to a smaller bike then adjust your new Horizon accordingly.
BTW by using a VK saddle adaptor, choosing from a multiplicity of stems and using the height adjustments already on the bike you should be able to get almost any position you want - backwards, forwards, up, down, saddle, bars etc. These bikes only come in four sizes because they have enough adjustability.
Last edited by horizon
on Thu Jun 07, 2012 6:19 pm, edited 1 time in total.
Life is like riding a bicycle. To stay safe you must wear a helmet. (Albert Einstein)