Well, your colleague clearly wanted the booth. Hoping to be offered another
table at the time has some possibilities, but if this place is as popular
as you suggest, then there's the distinct possibility that there won't BE
another table. I wouldn't leave this to chance.
Your colleague doesn't seem to have thought this through enough to realize
you wouldn't fit (I'm stopping short of flat out calling her thoughtless
and insensitive for putting you in this position in the first place. Who
invites a fat person someplace then reserves seats the fat person won't fit
in? But, to be kind, maybe she never remembers that you're fat enough to
have to take these things into account.).
Is it possible for the table to be moved at all? Or is it bolted to the
floor? If it can move then you might be able to make more room in the
booth, if it can't, you just won't fit, period. You can't change the laws
of physics to avoid having to talk to your colleages about this.
I see two options as reasonable. Call the restaurant, indicate that you're
going to be with so and so party on X day, and that they have the booth
reserved and ask if there's any way they can accommodate you so you can sit
there. You might want to try this one first. Let the restaurant decide if
you'd be in the way, or if there's any way for them to help you out. If
they can, great, no problem and you just go normally. I recommend doing
this first because if it doesn't work, then you're going to have to do the
other option, and this will give you fodder for it.
The other option would be to tell your colleague that you've contacted the
restaurant about accommodating you at the booth and they can't. Let her
know that you really would like to go, but that you don't want your
limitation to interfere with her enjoyment, so you're willing to take a
raincheck on the dinner out so she and her husband can enjoy the booth
rather than getting seated at a table instead. Make it *her* choice, since
she's the one who invited you.
Just be really polite and straightforward but NOT APOLOGETIC. You do NOT
have to apologize for being big. It's very simple, blame the restaurant
(not in a nasty way, but it IS their fault). They can't accommodate you,
and they've created an atmosphere that can't be modified to fit fat people.
Nothing for you or her to apologize for (well, technically I'd hope SHE
would apologize for not thinking about this when she made the reservation
and putting you through having to deal with it, but... that's just me I
suppose. I think if you're inviting people somewhere, it's your job to make
sure they are comfortable

.
Good luck, and let us know what happens!
--
Siobhan Perricone
One trend that bothers me is the glorification of
stupidity, that the media is reassuring people it's
all right not to know anything.... That to me is
far more dangerous than a little ****ography
on the Internet. - Carl Sagan