Mature size on Forsythia of for most shrubs for that matter is greatly controlled by soil, fertility, moisture, etc. That said here are some Monrovia Forsythias and their estimated landscape size:
KUMSON KOREAN GREENSTEM FORSYTHIA 6 ft. tall, 5 ft. wide.
FIESTA FORSYTHIA 5-6 ft. tall and wide.
MAGICAL GOLD FORSYTHIA 5 ft. tall, 4 ft. wide.
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