Lets do a combo
#1 - as
@480sparky said, normal enlarger lens = normal lens for that film format, 75mm for 6x4.5
I would start with this lens.
If you can't find a 75, go with an 80 which might be easier to find.
#2 - as
@webestang64 said, use a longer focal length for smaller prints.
When I used a 50mm lens to make small prints from 35mm film, the enlarger head was so close to the easel that I sometimes could not open the easel.
The other problem on some enlargers is, you can't get the head down close to the baseboard. I had to use a box to raise the easel. And I had no idea if the top and bottom surfaces of the box were PARALLEL. It probably wasn't parallel.
Both these situations were taken care of with a longer 75mm lens, in place of the normal 50mm lens. So I did not have to put the enlarger head so low.
For 6x4.5 I would use something 1.5x the normal lens.
So around 105mm as an alternate for the 75.
Today, I would look for GOOD lenses. Don't settle for 'economy' lenses.
You can find EXCELLENT lenses for very affordable prices.
I replaced my old "economy" set of lenses with a set of El-Nikkor lenses, all off
eBay for a good price. You just have to shop around.
Warning, the German lens names are confusing. There is a lower and higher line.
Schneider has Comonar (AR) and Componon (NON). You want the better CompoNON lens. Better yet, the Componon-S lens.
Rodenstock has a similar sounding pair of lens lines. Rodagon and ? It has been a long time since I researched that brand.
Caution, you NEED to pay attention to the mounting thread size of the lens.
MOST lenses have a 39mm mount.
But every once in a while you run into an odd size. I have one lens with a 32mm mount (came with the enlarger), which I later replaced with a lens with a standard 39mm mount.
When you get into the 4x5 lenses, 135 and 150mm, you need to be careful. The mounting flange for some of those lenses is LARGER than the standard 39mm. You will need to get a matching lens board for them. Or just make a 39mm mount part of your shopping requirement.
The El-Nikkor 135 has a 39mm mount. At least mine does.