Ah, your basic wheel & tire question. Your LBS should be able to answer that. But since changing a tire isn't hard and is a useful skill (!), you can change your tires yourself or at least select them yourself. Hmm, how much do we know about your rims? Have you told us? I don't recall.

The rims may give a size or say what kind of tires to use. An older 27" rim is probably marked 27" but a newer one may be marked 630 or ISO-630 or similar. The more modern size, which is the traditional size for high-end wheels, is 700c. They are extremely similar to 27" but the tires are NOT interchangeable. Those fake tires already on it may be marked with a size too. Anyway, whether the rim is 27" or 700c the tire width question is the same. And speaking of tire sizes, widths are often given in mm so that 1-1/4, 1-1/8, and 1" would be labelled 32, 28, and 25mm, respectively.

Those tire sizes aren't so different from each other so the simple answer is that it would be safe to pick the 1-1/8" (28mm). Paselas tend to be labelled accurately (which means they are wider than other makes for the same size because most brands are labelled bigger than they are). 28mm or even 32mm is what probably came new on that bike.

You can go beyond the simple answer with the following considerations. There is a lot to say but you can just jump to the end if you wish:

1. Check the rim width. If the outer width is bigger than, say, 26mm, a 1" tire may seem small but still work okay. If the outer width is smaller than, say, 20mm, a 1-1/4" tire may seem too large though it may still work.

2. A tire wider than the rim won't fit between the brake pads easily when you take a wheel off unless you release the brake a little or force the tire through. This argues for a narrower tire. But it's a minor inconvenience most of the time.

3. A narrower tire will be lighter which means it will feel more responsive when you are trying to accelerate quickly. However that's not really important unless you are competing (or trying to keep up with a bunch of hotshots).

4. A narrower tire will require higher pressure and ride harder. (Note, tires list a maximum pressure on their side but you don't have to run them that high.)

5. It has been argued that on rougher roads a wider tire has less rolling resistance. The physics would seem to support this, but at road speed wind resistance is so much higher that rolling resistance is a moot point.

6. The ideal tire depends on the typical roads in your area. If the roads are rougher a wider tire wiill roll more easily and fatigue you less on a long ride. Most importantly, with a wider tire you are much less likely to bottom out a rim (especially the rear) on a hard bump or pothole, a catastrophic event which can blow a tire and even damage a rim.

7. A heavier rider should run higher pressures to protect the rim, and maybe run wider tires for the same reason.

One final consideration is what the inside of the rim looks like. Newer rims have a "hooked bead" on the inside. This means that the outer edge of the rim protrudes inward compared to the rim's main sidewall. That bead provides a lip which the tire can seat against. Older rims may be straight-walled on the inside. Some people say you shouldn't run high pressures with a non-hooked-bead rim because the tire can pop off. (When it does it makes a LOUD bang! Like a gunshot.) How high is high? Well, my Pasela TG's are 27x1" and they show 115psi as the max. I've run them on an old straight-wall front rim all summer at 105 to 110psi, once at 115. I pumped them to 120 once (assuming my gauge was accurate), rode for an hour, and an hour later it blew. I don't pump them that high any more. But I intend to rebuild that wheel with a new hooked-bead rim. Don't do as I do...

This matters to you only if you run higher pressures. But your frame size is small so I expect you don't weigh much.

After all that, 1-1/8" would be conservative. I started the summer with 1-1/8" then switched to 1". I think I can feel the difference but I can't say it is any particular improvement. I like how it feels so I have no reason to go back to 1-1/8", but I would have been happy to keep the 1-1/8".

Finally, note that the Panaracer Pasela comes in two varieties, Pasela and Pasela TG (Tour Guard). The TG has an aramid or Kevlar belt to help prevent punctures. They are a little heavier than the non-TG and more expensive.