Size of old road frames compared to new ones May I use an old frame (pre 1990) of same size as a modern road frame? For example by using a long stem and long seatpost. Or would I have to size up a lot? 

Concrete example: I ride a 56 cm Merida CX frame (2017 Merida CX600) and am 190 cm tall. I found a nice 56 cm Motobecane steel frame (about 1985) on an online auction site.
Historically manufacturers have used different reference points in specifying frame measurements.  For example, some manufacturers use the length of the top tube while others use the length of the seat tube. If the frame is 'square', then the two measurements will be roughly the same, but this is not always the case.

The endpoints of the tube measurement may vary, especially with regards to the seat tube.  Typically the lower reference is the center of the bottom bracket.  The top reference, however, may be the center of the top tube (where it intersects the seat tube, aka 'center-to-center' or C-C), or it may be the top of the top tube (C-T), or it may be the top of the seat tube itself (C-TTT).  Sloping top tubes complicate this measurement in that the reference is typically relative to an imaginary, horizontal top tube and where it would intersect the seat tube.

The length of such an imaginary, horizontal top tube is sometimes specified as 'effective top tube length', or 'ETT'.

To reduce manufacturing costs and ease the sales process, manufacturers also have moved away from numeric sizing to 'XS', 'S', 'M', 'L', 'XL', sizing and  nomenclature.  The numeric measurements of a given size vary between manufacturers and even between bikes from one manufacturer, depending on the style of bike - 'race', 'endurance', 'gravel', 'adventure', 'touring', etc.

Fortunately there seems to be some convergence around a common set of measurements, including effective TT length, ST length (C-C), head tube length, bottom bracket (BB) drop or height, seat tube angle, head tube angle, and chainstay length.  Perhaps most important are two measurements known as 'stack' and 'reach', these being the vertical distance from the BB center to the stop of the head tube (typically exclusive of headset, but not always), and the horizontal distance from the BB center to the top of the head tube.  These measurements have a strong influence on the position of your torso, head, and arms, because they establish a baseline for the position of the handlebars and their relation to the saddle (including the saddle-to-handlebar vertical distance, or drop).

A modern geometry chart will put a picture to all this.

I think your basic question is 'Will a 56cm Motobecane fit like a 56cm Merida CX frame?'  Perhaps a better question is 'Can I make a 56cm Motobecane fit like my 56cm Merida CX?'   This assumes you are happy with the fit of your Merida, of course.  While you don't state your proportions (inseam, sleeve length, etc.), at 190cm a 56cm bike may be marginally too small for you.

To answer your question, one thing to do would be to compare the numeric geometry of the two frames.  A simple pass / fail test would be to measure the distance from the saddle nose to the handlebars, and the saddle-to-handlebar drop.  If you think you can make those measurements the same, or close to the same, then all other things being equal then the Motobecane may work for you. 
Note that any difference in BB drop will be offset by an change in saddle height to achieve the same leg extension, which in turn will affect the saddle-to-bar drop and the reach.

Cheap, used bikes are plentiful, at least where I am, so if the Motobecane cannot be made to work (via seatpost and/or stem and/or handlebar adjustment or replacement) then you should keep looking.