I have a very nice giant trinity which I don’t really want to use on the turbo all winter, so I bought an old cannondale slice frame specifically for turbo duties. It’s arrived and it looks bloody tiny!

The trinity is a small, geo Here

The slice is a 51, geo Here

Reach is my main concern, I can sort stack easy enough. Difference is 2cm between the frames, however placing both frames side by side the (unbuilt) slice looks tiny.

I had assumed a 2 cm longer stem in the slice would sort the issue, but is my thinking all wrong?

Ta

No, it’d work if it’s just for turbo use where handling isn’t a concern. The dimensions from bar to saddle to crank are all you need to get matched up.
edit to add, for shorter HI sessions it might not really matter anyway, just get it very roughly the same. My turbo bike doesn’t match my road or XC bikes fit but I’m (or was!) never on the turbo more than 45 mins at any real effort level, mainly leg effort not upper body etc

You’re right, it’ll be fine, it’s for a turbo so you could stick a 200cm stem on for all it matters if that feels comfy.

In theory you’re looking for 25mm though as it’s the TT you want to match.

There were some 105 5800 Slices on Evans for ~£600 earlier this year and the geo compared to my “58”cm Cube Attain confused the heck out of me!

Out with the tape measure ! Compare both, then adjust as necessary (e.g. longer stem).

Start with getting saddle height and setback from bottom bracket the same. Then bars height and reach.