I am getting looking for a vintage (1980s) steel frame to put together a city bike. I have a size 58cm Trek road bike now, but I need one I can lock up outside.
I am putting straight handlebars on it.
Do I get the same size or, because the bars are straight and therefore I won't be stretched out to the hoods, do I get a longer top tube so the bars are farther away?
I'm not sure I'm being clear here, but the issue is with the straight bars is the frame sizing the same?
thanks

There are different length stems.

if you don't spend all of your time in the front part of the drops, a straight bar will put you in about the same position as the tops of road bars. There isn't much of a fit issue. Make the seat fit your knees and legs; make the stem fit your upper body. Seems like bikes with shorter top tubes are older, and the trend has been to lengthen as they get newer - so the top tube will kind of depend on what age frame you buy.

Luker,
I ride most of my time on the hoods. That would put me closer on the city bike unless I get a larger frame with a resultant longer top tube. Or does it not matter as I just get a longer stem?
thanks

Frankly, I'd go with different bars. The straight bars don't allow much in the way of hand positions. You might try moustache bars or the Nitto Dove/Albatross. Nashbar's cheapie moustache bars are pretty good. Also, it's not a bad thing to be a bit more upright on a city bike/cruiser, for vision purposes. Stem length? Go a bit shorter than usual on moustache bars.

Also, I advise using two locks, even on a beater.

Poque is right; the hand positions on a straight bar are pretty limiting, but I kinda like 'em...Stem length is arbitrary within a range...a 95 and a 110 mm stem is gonna handle about the same, but a 70 and a 145 will feel wildly different. I think that the longer stem gives a greater lever arm and amplifies the motion of the handlebars. Longer stem = quicker steering, when the difference is significant. I think...I think I need to draw a free body diagram to get this straight...hold on...

nah. Had to resort to calculus and a lengthy discussion with my wife, the mechanical engineer. Longer stems slow down the steering...Shorter stems have more effect on the front wheel.