Where can I configure cache time and size of the pi-hole?
Right here: /etc/dnsmasq.d/01-pihole.conf 
That file can and often will be overwritten with Pi-hole updates. Need to check and see if a second config snippet /etc/dnsmasq/99-overrides.conf will override the config flags in 01-pihole.conf. Will update.
It seems that an override will throw the repeated keyword error on restart of dnsmasq. Can I ask why you would need to boost the values from their current settings @Martha_Martensen?
The cache value is already at the max of 10,000. @aqqlejak's solution should work, but at the moment, it will get overwritten on update/install.
I would also want to override this, for privacy settings. I want to make as little DNS requests as possible.
For example, the UK government asks ISP's to store DNS requests.
What I really want is a local copy of the entire DNS data for an entire country + the .com toplevel on my Raspberry Pi-Hole