presistor is for representing parasitics, their layout is what
you made by hooking stuff up.
You don't get to have all of the choices you'd like.
My recommendation would be high resistivity (unsilicided,
in-situ doped lightly) poly if this is an option in your
specific flow-branch. Well resistors are fat and will have
"side leakage" (junction leakage) and also a body effect
that varies along the chain, as well as a well-substrate
breakdown (may be a concern, depending on what you
want those megohms for.
For small signal and no great linearity concern, you might
bias a small long channel FET to get the right answer.
Probably a replica-bias scheme involving a smaller resistor
and larger (finger count) FET, to set it up.
Otherwise, 1K/sq poly and 10K squares, and like it. Despite
fairly poor tolerances and tempco and grain size effects
and noise attributes.