Could you please tell me how to increase the font size of my parameters and font size of correlation factors?
Do you think if it is possible to title x- and y- axes?
I'm not too familiar with base R plotting anymore, but is the pairs function you're using graphics::pairs?
The code you'r providing looks like one of the examples in that link. I think the key line is here:
if(missing(cex.cor)) cex.cor <- 0.8/strwidth(txt)
Since the function you're defining, panel.cor, doesn't receive any argument for cex.cor, so it's defaulting to 80% of strwidth(txt). You could try increasing that multiplier if you're fine to hard code it. I'm not sure whether you can just provide the argument in the call to pairs, like:
upper.panel = panel.cor(cex.cor = 16)
Try it and see (or maybe someone else can confirm)!