My favorite and also the first of the modern slim fountain pens is still the Aurora Hastil. In the 70s most makers had one or more slim cylindrical fountain pens. Others I really love are from Caran d'Ache, Parker, Waterman and ST Dupont.
Parker 45 or 95. Both are slim and in the same price range as the metropolitan. Both are no longer in production, but are available on the second hand market.
I like slim pens myself, although I have average sized hands for a man. Once you get the idea that it's not necessary to hold them in a death grip to keep them from getting away, light, slim pens are easy to control, and to use for long writing sessions.
The slimmest fountain pen with which I have personal experience is a Parker 180. Although no longer made, there are plenty of them out there, and I was able to buy one which was claimed to be unused, and quite possibly was (unused). Expect to pay in the low $100 range for a gold nibbed one in good condition, and look for one that includes the original squeeze converter. Although many fountain pens will write perfectly well and a bit finer when the nib is turned upside down, the 180 is actually designed for this. The one I have is broad with the nib right side up, and fine with it flipped over. Of course, you'd actually be buying one from someone who sells used pens, but I won't give that kind of link here. Here is some information on them, though. Parker Pens Penography: PARKER 180 - http://parkerpens.net/parker180.html.
For a pen that is still made and less expensive, look at the Faber Castell Neo, which I think will set you back about $40. Not quite as slim as the 180, but still slimmer than most, and quite light. The company is German but this lower end model is made in China.
And the Pilot Prera is pretty slim and not over long. I could recommend that one as well. A little more expensive than the entry level Metropolitan, but still very affordable, and in my opinion more attractive and definitely more comfortable in the hand.
Sheaffer Snorkels are quite slim and excellent pens. The Sheaffer Touchdown TM is basically the same pen, but filled through the nib instead of a snorkel.
For modern pens, tough to beat the Pelikan M200/205 for a slender pen.
Model :Silverbell. This is an EXCEPTIONAL Pilot fountain pen. Here are the facts about this pen Pilot Fountain Pen. Manufacture :Pilot. Nib size :Fine. Nib material :Stainless steel. You'll always get great service and great products.
Sheaffer Sagaris is a possibility. It looks to be about $10 more expensive than the Pilot. It’s also a friction fit cap and is metal. It has a nice smooth transition from the barrel to the section so you don’t have to deal with that huge, harsh step down that the Pilot has. I generally like somewhat thicker pens, but I would take the Sagaris any day over that step down.
Another one - I'm using my Cross Townsend today and its quite slim. The Cross Century is even slimmer. Cross is often dismissed as fuddy duddy by the smart set, but I really enjoy mine.