How to know what's the right Tamper diameter?

Selection guide for Tamper base diameter 

Home Espresso Machines Suggested Diameter
Ascaso 57 mm
Bodum Granos 51 mm
Breville (800 Series) 51 mm
Breville (Barista Express, BES870XL & BES860XL) (Infuser, BES840XL) (Duo-Temp Pro, BES810BSS) 53 mm
Breville (Dual Boiler, BES900XL & BES920XL) 58 mm
Cuisinart 51 mm
Delonghi (new) 51 mm
Delonghi (Old) 49 mm
ECM Botticelli 58 mm
ECM Cellini, Giotto 58 mm
Espressione Cafe Retro, Cafe Novecento 58 mm
Estro 53 mm
Faema Family 53 mm
Francis!Francis! X1 (new versions) 56 mm
Francis!Francis! X1 (old versions) 52 mm ou 53 mm*
Francis!Francis! X3 57 mm
Gaggia (semi-automatiques) 58 mm
Gaggia Factory (lever models) 51 mm
Innova 57 mm
Kitchen Aid Proline 58 mm
Krups 49 to 52 mm*
La Pavoni Club Combo, Duet, Napolitana 57 mm
La Pavoni lever machines (pre-1999) 51 mm
La Pavoni lever machines (post-1999) 49 mm
La Pavoni Lusso, Pisa, Si 53 mm
La Riveria 45 mm
La Spaziale 53 mm
La Valentina 58 mm
Mokita 57 mm
Mypressi Twist 53 mm
Olympia Club, Cremina, Express 49 mm
Pasquini Livia 90 58 mm
Quick Mill 58 mm
Rancilio Silvia 58 mm
Saeco 53 mm
Solis 53 mm
*Please see your manufacturer before placing your order.
Commercials Espresso Machines
Suggested Diameter
Astoria 58 mm
Astra 56 mm
Aurora 56 mm
Azkoyen 58 mm
Bezzera 58 mm
Brasilia 58 mm
Bunn 58 mm
Caramali 58 mm
Cimbali (others) 57 mm
Cimbali Junior Casa DT1 58 mm
Conti 58 mm
ECM 58 mm
Elektra 58 mm
Expobar 58 mm
Faema 58 mm
Florenzato 58 mm
Futurmat 58 mm
Gaggia 58 mm
Grimac 58 mm
Isomac 58 mm
La Marzocco 58 mm
La Scala 58 mm
La Spaziale 52 mm
Nuova Simonelli 58 mm
Olympia Cremina 49 mm
Pasquini 58 mm
Rancilio 58 mm
Reneka 56 mm
San Marco 55 mm
Spaziale 52 mm
Synesso 58 mm
Unic 56 mm
VFA 58 mm
Wega 58 mm