Size should not be an issue. Many small schools.
The fundamental problem with the survival of HBCUs is funding and finacial support from the alumni, (Negro )corporate America and the black community in general through the UNCF et al organizations. I would say that with the condition that these orgs like UNCF be heavily policed to see to it that the maximum amount of funds given and raised get to where it needs to be which is to the students going to these HBCUs and perhaps to the universities themselves for infrastructure and capital improvements.
I feel in addition to Negros getting off our butts and supporting our schools in a concrete way, financially, these institutions bare a responsibility to be fiscally sound and "run a taught ship". They must also innovate and think out of the box in order to survive.
Take my undergraduate institution, Tuskegee University, for example. The biggest liability that Tuskegee faces right now, besides financial support from Alumni/friends/corporate, is its location and the city it is in. Back in the old days nobody cared about where Tuskegee was. You just got your arse off to college and didn't come back until you graduated. Well, now days with all the potential students in the big metro areas across the country and all of the pop culture of today, selling a student on coming to Tuskegee right smack-dab in the middle of one of the poorest counties and poorest regions of the nation IS INCREASINGLY A PROBLEM. Many students who go to Tuskegee have some kind of ties rather its a family tradition or they are from large metro areas outside the south and want the HBCU experience. We get some students from Alabama, but we are getting our brains beat out in the recruitment battle against the likes of UAB, FREAKING AUM(here in Montgomery), Auburn, Troy State, U of Alabama, U OF SOUTH ALABAMA and UAH just to name a few. The reasoning ranges from tuition cost to image (many of these parents I talk to at work say, "Tuskegee/ASU ain't nuttin' but a party school. iz sendin' my yung'un to South Alabama because das were her friends is going,,, and dey beez a whole lots cheaper tuition dan Tuskegee; TUSKEGEE TOO HIGH!!"). Whatever the case, we are not competing very well with the other colleges in fundamental areas of infrastructure, campus amenities, etc (although we have made considerable improvements since the 80s).
Tuskegee University must partner, also, with the city of Tuskegee. The city of Tuskegee has become a huge liability for the University. The town and the county are simply stagnant and having nothing to offer the students at Tuskegee nor the faculty. All it does is leech off the economic windfall of having a university there. There is nothing about the city itself that helps attract students to the college. There is no industry to speak of there that helps build a community around the college to help attract top notch instructors/professors/deans. Most of them live in either Auburn or Montgomery. This can slowly kill a University if you can't continue to attract the leadership necessary to keep the university moving forward. Thus Tuskegee must partner with the city of Tuskegee and the state to push economic development in Tuskegee to help expand the economy there and create an environment that people and students want to live in which would enhance Tuskegee's appeal to students and faculty. Same with these other small HBCUs especially if they are in rural small town areas like Tuskegee.