I've developed circuit with Cyclone-5 (5CGXFC5M11) .I have a very little place on the and trying to reduce every component .
Is it possible to replace standard Flash memory IC EPCS64SI16N (SOIC-16) by another , much smaller Flash memory IC? What alternative device can you
EPCS64 series is obsolete, no longer in production.
I would look at ISSI: http://www.issi.com/WW/pdf/25LP-WP064D.pdf which is available in other much smaller package options than SOIC-16
Contact ISSI and ask for their app note AN25R001
I would look at Micron MT25Q device family: https://www.micron.com/products/nor-flash/serial-nor-flash/mt25q
The mentioned family seems to be validated and supported in SW by Intel:
We used Winbond W25Q128JVPIQ series on a Cyclone V GX board and it worked. Make sure it is the IQ suffix. You must use Quartus 17.1 or later and select, in this case, EPCQ128A in AS x4 when you generated the programming file. Of course for different sizes replace 128 with your size.