Vintage Peugeot conversion help and tips please

hey

lots of peoples been mentioning building a rear wheel. can someone explain why? is it necessary when converting a road bike?

because you would need a new hubs that designed for fixed wheel (if you're keeping the original rims and spokes that come with the bike), the road one are designed both for freewheel and to hold numerous cogs, so it's not suitable for fixed wheel.

also for those of you with rear drop outs, how did you know what chain ring size to ensure tension? experience.?? I got a 61cm raliegh (not enough representation on this forum to my mind!) ive stripped down an want to convert to a fixie on the cheap but i nervous about buying chains off t'internet in case proves unrideable/ not tight enough.
cheers

by rear drop out, you mean those drop out that doesn't allowed you to adjust the tightness of the chain?