Japanese Gardens are way bigger. But they are up in Washington Park so they are kind of an impractical location. It's do-able, but a bit of a challenge. Chinese Gardens are in Chinatown right in downtown, however.
Here are the links to their websites.
http://www.japanesegarden.com/http://www.portlandchinesegarden.org/Japanese GardenHours
Tuesday - Sunday: 10am - 4pm
(Mondays: 12pm - 4pm)
Last admission taken at 3:30pm
Admission
Adults $6.75
Seniors $5.00 (62 & over)
College Students $5.00 (College ID is required)
Students (6-17) $4.00
Children five and under are free
611 SW Kingston Ave
Portland, Oregon 97201
503-223-1321
Chinese GardenHours
April - October, 9AM to 6PM
November - March, 10AM - 5PM
Admission
General: $7 Seniors: $6 Students: $5.50 Free to children 5 and younger.
Portland Classical Chinese Garden
NW 3rd and Everett, Portland, OR 97209
503-228-8131