How can I tell the size of a pen?

Each pen has the dimensions (and in some cases weight too) detailed on their individual webpage. Look for the "Dimensions" section under the images and description.

Here are some detailed dimensions of popular pens. More will be added as we go on.
 

Pen modelnib lengthsection length to threads*section diameter max/minbarrel diameter maxbarrel/section lengthbarrel plus posted cap lengthweight
Aurora Talentum22.22210.6/10.013.9110137
Bexley BX80222.92110.7/9.512.9102133
Bexley Model 5823.418.811.5/10.513.9108164
Bexley Poseidon23.416.511.7/11.513.699151
Bexley Imperial23.421.611.1/9.914.9113-
Caran d'Ache Dunas16.623.59.9/7.412.4105135
Caran d'Ache Leman19.12110.5/8.913.310646
Conklin Mark Twain22.82111.6/9.612.310714627
Conway Stewart 5819.41910.5/8.611.9101142
Conway Stewart 10023.31711.3/10.313.1103144
Conway Stewart Capulet23.318.810.0/9.310.710815112
Conway Stewart Collector's Edition18.51910.9/9.013.2107141
Conway Stewart Churchill23.32612.8/10.814.711515730
Conway Stewart Duro23.32011.4/10.012.710814820
Conway Stewart Brunel23.322111-84
Conway Stewart Belliver23.31710.1/11.313.311414236
Conway Stewart Raleigh23.31911.4/10.313.511715648
Conway Stewart Wellington23.32011.3/9.913.3105153
Conway Stewart Wordsworth23.310.8/8.812.9102141
Lamy 20006.43210.8/6.412.511614322
Lamy 2000M6.43210.8/6.412.511614354
Lamy Safari17.035.512.1/9.612.211214917
Lamy AL-Star/Lx17.035.512.1/9.612.211214920
Lamy Studio17.03011.6/8.512.311213831
Lamy dialog 316.3-13.513.5139-46
Parker Duofold Centennial23.22110.9/9.81310515028
Pelikan M200/M215/M250/M40020.014.99.8/9.111.810212914
Pelikan M60020.614.910.5/9.612.510313418
Pelikan M80022.615.511.3/10.513.310513930
Pelikan M10002717.912.1/11.4 14.334
Sailor 191122.31711.6/10.313.210213118
Sailor 1911 Realo22.31811.6/10.313.29813221
Sailor King of Pen26.517.513.0/12.115.310636
Sailor King Professional Gear26.517.513.0/12.115.210913536
Sailor Pro Color/Shikiroi Hisakata19.616.810.3/9.412.19713716.3
Sailor Professional Gear22.31711.6/10.313.29412720
Sailor Sapporo/ Pro Gear Slim19.91710.3/9.312.09012318
TWSBI Diamond 58018.22110.0/9.313.111115729
Yard-O-Led Retro Grand233012.0/9.31411414666
Yard-O-Led Retro standard17.1238.6/7.610.910514224
Yard-O-Led Viceroy standard17.3248.9/7.69.610613932
Yard-O-Led Viceroy Grand233012.0/9.313.211666
Yard-O-Led Viceroy pocket17.668.9/7.29.87711026
Yard-O-Led Corinthian17.5249.9/7.511.3108-52
* on pens with a clip-on cap (eg Yard-O-Led and Lamy) this measurement is to the part where the cap clips on

 

Note: the above figures are actual measured dimensions in mm and weight in g. They may differ slightly from the manufacturer's published figures.

 

Below are images of some popular fountain pens for a visual size comparison.

 

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