How do I change the size of raw text?

[Andrew]

@Tobias_Andersson_Gid

However, I really do want to have it as a function so that I can control the language and the size of the source code (as well as being easier to just copy-and-paste from the editor to typst)

Here is a conventional-ish way, if you want to pass the code verbatim to the function:

#import "@preview/codly:1.2.0": *
#import "@preview/codly-languages:0.1.8": *

#let code(body, sz: 19pt) = {
  show: codly-init
  codly(stroke: 1pt + black, languages: codly-languages)
  show raw: set par(leading: 0.65em)
  show raw: set text(size: sz)
  body
}

#code(```py
a = 5
b = "a"
print(a, b)
```)

Or if you really want to set the language as a separate argument:

#import "@preview/codly:1.2.0": *
#import "@preview/codly-languages:0.1.8": *

#let code(body, sz: 19pt, ..args) = {
  show: codly-init
  codly(stroke: 1pt + black, languages: codly-languages)
  show raw: set par(leading: 0.65em)
  show raw: set text(size: sz)
  raw(body.text, block: true, ..args)
}

#code(lang: "py", ```
a = 5
b = "a"
print(a, b)
```)